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# 📚 PyTorch Tutorial for Beginners - Part 1/6: Fundamentals & Tensors
#PyTorch #DeepLearning #MachineLearning #NeuralNetworks #Tensors

Welcome to Part 1 of our comprehensive PyTorch series! This beginner-friendly lesson covers core concepts, tensor operations, and your first neural network.

---

## 🔹 What is PyTorch?
PyTorch is an open-source deep learning framework developed by Facebook's AI Research Lab (FAIR). Key features:

✔️ Dynamic computation graphs (define-by-run)
✔️ GPU acceleration with CUDA
✔️ Pythonic syntax for intuitive coding
✔️ Automatic differentiation (autograd)
✔️ Rich ecosystem (TorchVision, TorchText, etc.)

import torch
print(f"PyTorch version: {torch.__version__}")
print(f"CUDA available: {torch.cuda.is_available()}")

---

## 🔹 Tensors: The Building Blocks
Tensors are PyTorch's multi-dimensional arrays (like NumPy but with GPU support).

### 1. Creating Tensors

# From Python list
a = torch.tensor([1, 2, 3])  # 1D tensor (vector)

# 2D tensor (matrix)
b = torch.tensor([[1., 2.], [3., 4.]])

# Special tensors
zeros = torch.zeros(2, 3)      # 2x3 matrix of zeros
ones = torch.ones_like(zeros)  # Same shape as zeros, filled with 1s
rand = torch.rand(3, 3)        # 3x3 matrix with uniform random values (0-1)

### 2. Tensor Attributes

x = torch.rand(2, 3)
print(f"Shape: {x.shape}")      # torch.Size([2, 3])
print(f"Data type: {x.dtype}")  # torch.float32
print(f"Device: {x.device}")    # cpu/cuda:0

### 3. Moving Tensors to GPU

if torch.cuda.is_available():
    x = x.to('cuda')  # Move to GPU
    print(f"Now on: {x.device}")  # cuda:0

---

## 🔹 Tensor Operations
### 1. Basic Math

x = torch.tensor([1., 2., 3.])
y = torch.tensor([4., 5., 6.])

# Element-wise operations
add = x + y        # or torch.add(x, y)
sub = x - y
mul = x * y
div = x / y

# Matrix multiplication
mat1 = torch.rand(2, 3)
mat2 = torch.rand(3, 2)
matmul = torch.mm(mat1, mat2)  # or mat1 @ mat2

### 2. Reshaping Tensors

x = torch.arange(6)          # [0, 1, 2, 3, 4, 5]
x_reshaped = x.view(2, 3)    # [[0, 1, 2], [3, 4, 5]]
x_flattened = x.flatten()    # Back to 1D

### 3. Indexing & Slicing

x = torch.tensor([[1, 2], [3, 4], [5, 6]])
print(x[0, 1])    # 2 (first row, second column)
print(x[:, 0])    # [1, 3, 5] (all rows, first column)

---

## 🔹 Autograd: Automatic Differentiation
PyTorch automatically computes gradients for tensors with requires_grad=True.

### 1. Basic Example

x = torch.tensor(2.0, requires_grad=True)
y = x**2 + 3*x + 1
y.backward()      # Compute gradients
print(x.grad)     # dy/dx = 2x + 3 → 7.0

### 2. Neural Network Context

# Simple linear regression
w = torch.randn(1, requires_grad=True)
b = torch.zeros(1, requires_grad=True)

# Forward pass
inputs = torch.tensor([[1.0], [2.0], [3.0]])
targets = torch.tensor([[2.0], [4.0], [6.0]])
predictions = inputs * w + b

# Loss and backward pass
loss = torch.mean((predictions - targets)**2)
loss.backward()  # Computes dloss/dw, dloss/db

print(f"Gradient of w: {w.grad}")
print(f"Gradient of b: {b.grad}")

---

## **🔹 Your First Neural Network**
Let's build a single-layer perceptron for binary classification.

### 1. Define the Model

import torch.nn as nn

class Perceptron(nn.Module):
    def __init__(self, input_dim):
        super().__init__()
        self.linear = nn.Linear(input_dim, 1)  # 1 output neuron
        
    def forward(self, x):
        return torch.sigmoid(self.linear(x))  # Sigmoid for probability

model = Perceptron(input_dim=2)
print(model)

### 2. Synthetic Dataset

# XOR-like dataset
X = torch.tensor([[0, 0], [0, 1], [1, 0], [1, 1]], dtype=torch.float32)
y = torch.tensor([[0], [1], [1], [0]], dtype=torch.float32)

### 3. Training Loop

criterion = nn.BCELoss()          # Binary Cross Entropy
optimizer = torch.optim.SGD(model.parameters(), lr=0.1)

for epoch in range(1000):
    # Forward pass
    outputs = model(X)
    loss = criterion(outputs, y)
    
    # Backward pass
    optimizer.zero_grad()  # Clear old gradients
    loss.backward()        # Compute gradients
    optimizer.step()       # Update weights
    
    if (epoch+1) % 100 == 0:
        print(f'Epoch {epoch+1}, Loss: {loss.item():.4f}')

# Test
with torch.no_grad():
    predictions = model(X).round()
    print(f"Final predictions: {predictions.squeeze()}")

---

## 🔹 Best Practices for Beginners
1. Always clear gradients with optimizer.zero_grad() before backward()
2. Use `with torch.no_grad():` for inference (disables gradient tracking)
3. Normalize input data (e.g., scale to [0, 1] or standardize)
4. Start simple before using complex architectures
5. Leverage GPU for larger models/datasets

---

### 📌 What's Next?
In Part 2, we'll cover:
➡️ Deep Neural Networks (DNNs)
➡️ Activation Functions
➡️ Batch Normalization
➡️ Handling Real Datasets

#PyTorch #DeepLearning #MachineLearning 🚀

Practice Exercise:
1. Create a tensor of shape (3, 4) with random values (0-1)
2. Compute the mean of each column
3. Build a perceptron for OR gate (modify the XOR example)
4. Plot the loss curve during training

# Solution for exercise 1-2
x = torch.rand(3, 4)
col_means = x.mean(dim=0)  # dim=0 → average along rows

# 📚 PyTorch Tutorial for Beginners - Part 2/6: Deep Neural Networks & Training Techniques
#PyTorch #DeepLearning #MachineLearning #NeuralNetworks #Training

Welcome to Part 2 of our comprehensive PyTorch series! This lesson dives deep into building and training neural networks, covering architectures, activation functions, optimization, and more.

---

## 🔹 Recap & Setup

import torch
import torch.nn as nn
import torch.optim as optim
import matplotlib.pyplot as plt
from torch.utils.data import DataLoader, TensorDataset

# Check GPU
device = torch.device('cuda' if torch.cuda.is_available() else 'cpu')
print(f"Using device: {device}")

---

## 🔹 Deep Neural Network (DNN) Architecture
### 1. Key Components
| Component | Purpose | PyTorch Implementation |
|--------------------|-------------------------------------------------------------------------|------------------------------|
| Input Layer | Receives raw features | nn.Linear(input_dim, hidden_dim) |
| Hidden Layers | Learn hierarchical representations | Multiple nn.Linear + Activation |
| Output Layer | Produces final predictions | nn.Linear(hidden_dim, output_dim) |
| Activation | Introduces non-linearity | nn.ReLU(), nn.Sigmoid(), etc. |
| Loss Function | Measures prediction error | nn.MSELoss(), nn.CrossEntropyLoss() |
| Optimizer | Updates weights to minimize loss | optim.SGD(), optim.Adam() |

### 2. Building a DNN

class DNN(nn.Module):
    def __init__(self, input_size, hidden_sizes, output_size):
        super().__init__()
        layers = []
        
        # Hidden layers
        prev_size = input_size
        for hidden_size in hidden_sizes:
            layers.append(nn.Linear(prev_size, hidden_size))
            layers.append(nn.ReLU())
            prev_size = hidden_size
            
        # Output layer (no activation for regression)
        layers.append(nn.Linear(prev_size, output_size))
        
        self.net = nn.Sequential(*layers)
        
    def forward(self, x):
        return self.net(x)

# Example: 3-layer network (input=10, hidden=[64,32], output=1)
model = DNN(10, [64, 32], 1).to(device)
print(model)

---

## 🔹 Activation Functions
### 1. Common Choices
| Activation | Formula | Range | Use Case | PyTorch |
|-----------------|----------------------|------------|------------------------------|------------------|
| ReLU | max(0, x) | [0, ∞) | Hidden layers | nn.ReLU() |
| Leaky ReLU | max(0.01x, x) | (-∞, ∞) | Avoid dead neurons | nn.LeakyReLU() |
| Sigmoid | 1 / (1 + e^(-x)) | (0, 1) | Binary classification | nn.Sigmoid() |
| Tanh | (e^x - e^(-x)) / ... | (-1, 1) | RNNs, some hidden layers | nn.Tanh() |
| Softmax | e^x / sum(e^x) | (0, 1) | Multi-class classification | nn.Softmax() |

### 2. Visual Comparison

x = torch.linspace(-5, 5, 100)
activations = {
    "ReLU": nn.ReLU()(x),
    "LeakyReLU": nn.LeakyReLU(0.1)(x),
    "Sigmoid": nn.Sigmoid()(x),
    "Tanh": nn.Tanh()(x)
}

plt.figure(figsize=(12, 4))
for i, (name, y) in enumerate(activations.items()):
    plt.subplot(1, 4, i+1)
    plt.plot(x.numpy(), y.numpy())
    plt.title(name)
plt.tight_layout()
plt.show()

---

## 🔹 Loss Functions
### 1. Common Loss Functions
| Task | Loss Function | PyTorch Implementation |
|-----------------------|----------------------------|------------------------------|
| Regression | Mean Squared Error (MSE) | nn.MSELoss() |
| Binary Classification | Binary Cross Entropy (BCE) | nn.BCELoss() |
| Multi-class | Cross Entropy | nn.CrossEntropyLoss() |
| Imbalanced Data | Focal Loss | Custom implementation |

### 2. Custom Loss Example

class FocalLoss(nn.Module):
    def __init__(self, alpha=0.25, gamma=2):
        super().__init__()
        self.alpha = alpha
        self.gamma = gamma
        
    def forward(self, inputs, targets):
        BCE_loss = nn.BCEWithLogitsLoss(reduction='none')(inputs, targets)
        pt = torch.exp(-BCE_loss)
        loss = self.alpha * (1-pt)**self.gamma * BCE_loss
        return loss.mean()

---

## **🔹 Optimization Techniques**
### 1. Optimizers Comparison
| Optimizer | Key Features | Use Case |
|-----------------|-------------------------------------------|------------------------------|
| SGD | Simple, can get stuck in local minima | Basic models |
| SGD+Momentum| Accumulates velocity for smoother updates | Most scenarios |
| Adam | Adaptive learning rates | Default for many problems |
| RMSprop | Adapts learning rates per parameter | RNNs, some CNN architectures |

# Example optimizers
optimizer_SGD = optim.SGD(model.parameters(), lr=0.01)
optimizer_momentum = optim.SGD(model.parameters(), lr=0.01, momentum=0.9)
optimizer_Adam = optim.Adam(model.parameters(), lr=0.001)

### 2. Learning Rate Scheduling

# Step LR scheduler
scheduler = optim.lr_scheduler.StepLR(optimizer, step_size=30, gamma=0.1)

# Cosine annealing
scheduler_cosine = optim.lr_scheduler.CosineAnnealingLR(optimizer, T_max=100)

# Usage in training loop
for epoch in range(100):
    # Training steps...
    scheduler.step()

---

## 🔹 Batch Normalization & Dropout
### 1. Batch Normalization
Normalizes layer inputs to reduce internal covariate shift.

class DNNWithBN(nn.Module):
    def __init__(self, input_size, hidden_sizes, output_size):
        super().__init__()
        layers = []
        prev_size = input_size
        
        for hidden_size in hidden_sizes:
            layers.extend([
                nn.Linear(prev_size, hidden_size),
                nn.BatchNorm1d(hidden_size),
                nn.ReLU()
            ])
            prev_size = hidden_size
            
        layers.append(nn.Linear(prev_size, output_size))
        self.net = nn.Sequential(*layers)

### 2. Dropout
Randomly deactivates neurons to prevent overfitting.

self.net = nn.Sequential(
    nn.Linear(784, 256),
    nn.ReLU(),
    nn.Dropout(0.5),  # 50% dropout
    nn.Linear(256, 10)
)

---

## 🔹 Data Loading & Preprocessing
### 1. Using DataLoader

from torchvision import datasets, transforms

# Transformations
transform = transforms.Compose([
    transforms.ToTensor(),
    transforms.Normalize((0.5,), (0.5,))
])

# Load MNIST
train_data = datasets.MNIST('./data', train=True, download=True, transform=transform)
test_data = datasets.MNIST('./data', train=False, transform=transform)

# Create DataLoaders
train_loader = DataLoader(train_data, batch_size=64, shuffle=True)
test_loader = DataLoader(test_data, batch_size=64, shuffle=False)

### 2. Custom Dataset

class CustomDataset(torch.utils.data.Dataset):
    def __init__(self, X, y, transform=None):
        self.X = X
        self.y = y
        self.transform = transform
        
    def __len__(self):
        return len(self.X)
    
    def __getitem__(self, idx):
        sample = self.X[idx], self.y[idx]
        if self.transform:
            sample = self.transform(sample)
        return sample

---

## 🔹 Complete Training Pipeline
### 1. Training Loop

def train(model, train_loader, criterion, optimizer, epochs=10):
    model.train()
    losses = []
    
    for epoch in range(epochs):
        running_loss = 0.0
        
        for inputs, labels in train_loader:
            inputs, labels = inputs.to(device), labels.to(device)
            
            # Forward pass
            outputs = model(inputs)
            loss = criterion(outputs, labels)
            
            # Backward pass
            optimizer.zero_grad()
            loss.backward()
            optimizer.step()
            
            running_loss += loss.item()
        
        epoch_loss = running_loss / len(train_loader)
        losses.append(epoch_loss)
        print(f'Epoch {epoch+1}/{epochs}, Loss: {epoch_loss:.4f}')
    
    return losses

### 2. Evaluation Function

def evaluate(model, test_loader):
    model.eval()
    correct = 0
    total = 0
    
    with torch.no_grad():
        for inputs, labels in test_loader:
            inputs, labels = inputs.to(device), labels.to(device)
            outputs = model(inputs)
            _, predicted = torch.max(outputs.data, 1)
            total += labels.size(0)
            correct += (predicted == labels).sum().item()
    
    accuracy = 100 * correct / total
    print(f'Test Accuracy: {accuracy:.2f}%')
    return accuracy

### 3. Full Execution

# Hyperparameters
input_size = 784  # MNIST images (28x28)
hidden_sizes = [128, 64]
output_size = 10   # Digits 0-9
lr = 0.001
epochs = 10

# Initialize
model = DNN(input_size, hidden_sizes, output_size).to(device)
criterion = nn.CrossEntropyLoss()
optimizer = optim.Adam(model.parameters(), lr=lr)

# Flatten MNIST images
train_loader.dataset.transform = transforms.Compose([
    transforms.ToTensor(),
    transforms.Normalize((0.5,), (0.5,)),
    transforms.Lambda(lambda x: x.view(-1))  # Flatten
])

# Train and evaluate
losses = train(model, train_loader, criterion, optimizer, epochs)
evaluate(model, test_loader)

# Plot training curve
plt.plot(losses)
plt.xlabel('Epoch')
plt.ylabel('Loss')
plt.title('Training Loss Curve')
plt.show()

---

## 🔹 Debugging & Visualization
### 1. Gradient Checking

# After loss.backward()
for name, param in model.named_parameters():
    if param.grad is not None:
        print(f"{name} gradient mean: {param.grad.mean().item():.6f}")

### 2. Weight Histograms

def plot_weights(model):
    for name, param in model.named_parameters():
        if 'weight' in name:
            plt.figure()
            plt.hist(param.detach().cpu().numpy().flatten(), bins=50)
            plt.title(name)
            plt.show()

---

## 🔹 Advanced Techniques
### 1. Weight Initialization

def init_weights(m):
    if isinstance(m, nn.Linear):
        nn.init.xavier_uniform_(m.weight)
        nn.init.zeros_(m.bias)

model.apply(init_weights)

### 2. Early Stopping

best_loss = float('inf')
patience = 3
trigger_times = 0

for epoch in range(100):
    # Training...
    val_loss = validate(model, val_loader, criterion)
    
    if val_loss < best_loss:
        best_loss = val_loss
        trigger_times = 0
        torch.save(model.state_dict(), 'best_model.pth')
    else:
        trigger_times += 1
        if trigger_times >= patience:
            print("Early stopping!")
            break

---

## 🔹 Best Practices
1. Always normalize input data (e.g., scale to [0,1] or standardize)
2. Use batch normalization for deeper networks
3. Start with Adam optimizer (lr=0.001) as default
4. Monitor training with validation set to detect overfitting
5. Visualize weight distributions periodically
6. Use GPU for training (model.to(device))

---

### 📌 What's Next?
In Part 3, we'll cover:
➡️ Convolutional Neural Networks (CNNs)
➡️ Transfer Learning
➡️ Image Augmentation Techniques
➡️ Visualizing CNNs

#PyTorch #DeepLearning #MachineLearning 🚀

Practice Exercise:
1. Modify the DNN to have 4 hidden layers [256, 128, 64, 32]
2. Try different activation functions (LeakyReLU, Tanh)
3. Implement learning rate scheduling
4. Add dropout and compare results
5. Plot accuracy vs. epoch during training

# Sample solution for exercise 1
model = DNN(784, [256, 128, 64, 32], 10).to(device)

# 📚 PyTorch Tutorial for Beginners - Part 3/6: Convolutional Neural Networks (CNNs) & Computer Vision
#PyTorch #DeepLearning #ComputerVision #CNNs #TransferLearning

Welcome to Part 3 of our PyTorch series! This comprehensive lesson dives deep into Convolutional Neural Networks (CNNs), the powerhouse behind modern computer vision applications. We'll cover architecture design, implementation tricks, transfer learning, and visualization techniques.

---

## 🔹 Introduction to CNNs
### Why CNNs for Images?
Traditional fully-connected networks (DNNs) fail for images because:
- Parameter explosion: A 256x256 RGB image → 196,608 input features
- No spatial awareness: DNNs treat pixels as independent features
- Translation variance: Objects in different positions require re-learning

### CNN Key Innovations
| Concept | Purpose | Visual Example |
|--------------------|-------------------------------------------------------------------------|-----------------------------|
| Local Receptive Fields | Processes small regions at a time (e.g., 3x3 windows) |  |
| Weight Sharing | Same filters applied across entire image (reduces parameters) | |
| Hierarchical Features | Early layers detect edges → textures → object parts → whole objects |  |

---

## 🔹 Core CNN Components
### 1. Convolutional Layers

import torch.nn as nn

# 2D convolution (for images)
conv = nn.Conv2d(
    in_channels=3,    # Input channels (RGB=3, grayscale=1)
    out_channels=16,  # Number of filters
    kernel_size=3,    # 3x3 filter
    stride=1,         # Filter movement step
    padding=1         # Preserves spatial dimensions (with stride=1)
)

# Shape transformation: (batch, channels, height, width)
x = torch.randn(32, 3, 64, 64)  # 32 RGB images of 64x64
print(conv(x).shape)  # → torch.Size([32, 16, 64, 64])

### 2. Pooling Layers

# Max pooling (common for downsampling)
pool = nn.MaxPool2d(kernel_size=2, stride=2)
print(pool(conv(x)).shape)  # → torch.Size([32, 16, 32, 32])

# Adaptive pooling (useful for varying input sizes)
adaptive_pool = nn.AdaptiveAvgPool2d((7, 7))
print(adaptive_pool(x).shape)  # → torch.Size([32, 3, 7, 7])

### 3. Normalization Layers

# Batch Normalization
bn = nn.BatchNorm2d(16)  # num_features = out_channels
x = conv(x)
x = bn(x)

# Layer Normalization (for NLP/sequences)
ln = nn.LayerNorm([16, 64, 64])

### 4. Dropout

# Spatial dropout (drops entire channels)
dropout = nn.Dropout2d(p=0.25)

---

## 🔹 Building a CNN from Scratch
### Complete Architecture

class CNN(nn.Module):
    def __init__(self, num_classes=10):
        super().__init__()
        self.features = nn.Sequential(
            # Block 1
            nn.Conv2d(3, 32, kernel_size=3, padding=1),
            nn.BatchNorm2d(32),
            nn.ReLU(),
            nn.MaxPool2d(2),
            
            # Block 2
            nn.Conv2d(32, 64, kernel_size=3, padding=1),
            nn.BatchNorm2d(64),
            nn.ReLU(),
            nn.MaxPool2d(2),
            
            # Block 3
            nn.Conv2d(64, 128, kernel_size=3, padding=1),
            nn.BatchNorm2d(128),
            nn.ReLU(),
            nn.MaxPool2d(2),
        )
        
        self.classifier = nn.Sequential(
            nn.Linear(128 * 4 * 4, 512),  # Adjusted based on input size
            nn.ReLU(),
            nn.Dropout(0.5),
            nn.Linear(512, num_classes)
        )
        
    def forward(self, x):
        x = self.features(x)
        x = torch.flatten(x, 1)  # Flatten all dimensions except batch
        x = self.classifier(x)
        return x

# Usage
model = CNN().to(device)
print(model)

### Shape Calculation Formula
For a layer with:
- Input size: (Hᵢₙ, Wᵢₙ)
- Kernel: K
- Padding: P
- Stride: S

Output dimensions:

Hₒᵤₜ = ⌊(Hᵢₙ + 2P - K)/S⌋ + 1
Wₒᵤₜ = ⌊(Wᵢₙ + 2P - K)/S⌋ + 1

---

## 🔹 Transfer Learning
### 1. Why Transfer Learning?
- Leverages pre-trained models on large datasets (ImageNet)
- Requires less data for new tasks
- Faster training convergence

### 2. Using Pretrained Models

from torchvision import models

# Load pretrained ResNet18
resnet = models.resnet18(pretrained=True)

# Freeze all layers
for param in resnet.parameters():
    param.requires_grad = False

# Replace final layer
num_ftrs = resnet.fc.in_features
resnet.fc = nn.Linear(num_ftrs, 10)  # New task with 10 classes

# Only new FC layer will be trained
optimizer = optim.Adam(resnet.fc.parameters(), lr=0.001)

### 3. Feature Extraction Pipeline

from torchvision import transforms

# Preprocessing for pretrained models
preprocess = transforms.Compose([
    transforms.Resize(256),
    transforms.CenterCrop(224),
    transforms.ToTensor(),
    transforms.Normalize(
        mean=[0.485, 0.456, 0.406],  # ImageNet stats
        std=[0.229, 0.224, 0.225]
    )
])

# Create dataset
train_data = datasets.ImageFolder('data/train', transform=preprocess)
train_loader = DataLoader(train_data, batch_size=32, shuffle=True)

---

## 🔹 Data Augmentation
### 1. Common Techniques

train_transform = transforms.Compose([
    transforms.RandomResizedCrop(224),
    transforms.RandomHorizontalFlip(),
    transforms.RandomRotation(15),
    transforms.ColorJitter(brightness=0.2, contrast=0.2, saturation=0.2),
    transforms.ToTensor(),
    transforms.Normalize([0.485, 0.456, 0.406], [0.229, 0.224, 0.225])
])

### 2. Advanced Augmentations (Albumentations)

import albumentations as A
from albumentations.pytorch import ToTensorV2

transform = A.Compose([
    A.RandomRotate90(),
    A.Flip(),
    A.Transpose(),
    A.GaussNoise(p=0.2),
    A.OneOf([
        A.MotionBlur(p=0.2),
        A.MedianBlur(blur_limit=3, p=0.1),
        A.Blur(blur_limit=3, p=0.1),
    ], p=0.2),
    A.Normalize(mean=(0.485, 0.456, 0.406), std=(0.229, 0.224, 0.225)),
    ToTensorV2()
])

---

## 🔹 Training Tricks for CNNs
### 1. Learning Rate Finder

from torch_lr_finder import LRFinder

criterion = nn.CrossEntropyLoss()
optimizer = optim.Adam(model.parameters(), lr=1e-7)
lr_finder = LRFinder(model, optimizer, criterion, device='cuda')
lr_finder.range_test(train_loader, end_lr=10, num_iter=100)
lr_finder.plot()  # Identify optimal lr from plot
lr_finder.reset()

### 2. Mixed Precision Training

from torch.cuda.amp import GradScaler, autocast

scaler = GradScaler()

for inputs, labels in train_loader:
    inputs, labels = inputs.to(device), labels.to(device)
    
    with autocast():
        outputs = model(inputs)
        loss = criterion(outputs, labels)
    
    scaler.scale(loss).backward()
    scaler.step(optimizer)
    scaler.update()
    optimizer.zero_grad()

### 3. Gradient Clipping

torch.nn.utils.clip_grad_norm_(model.parameters(), max_norm=1.0)

---

## 🔹 Visualization Techniques
### 1. Feature Maps Visualization

def visualize_feature_maps(model, image):
    # Hook to get intermediate features
    features = []
    def hook(module, input, output):
        features.append(output.detach())
    
    # Register hook to first conv layer
    handle = model.features[0].register_forward_hook(hook)
    
    # Forward pass
    model(image.unsqueeze(0))
    handle.remove()
    
    # Plot first 16 filters
    fig, axes = plt.subplots(4, 4, figsize=(12, 12))
    for i, ax in enumerate(axes.flat):
        ax.imshow(features[0][0, i].cpu(), cmap='viridis')
        ax.axis('off')
    plt.show()

### 2. Grad-CAM (Class Activation Maps)

class GradCAM:
    def __init__(self, model, target_layer):
        self.model = model
        self.target_layer = target_layer
        self.gradients = None
        self.activations = None
        
        # Hook setup
        target_layer.register_forward_hook(self.save_activations)
        target_layer.register_backward_hook(self.save_gradients)
    
    def save_activations(self, module, input, output):
        self.activations = output.detach()
    
    def save_gradients(self, module, grad_input, grad_output):
        self.gradients = grad_output[0].detach()
    
    def __call__(self, x, class_idx=None):
        # Forward pass
        output = self.model(x)
        
        if class_idx is None:
            class_idx = output.argmax(dim=1)
        
        # Backward pass for specific class
        self.model.zero_grad()
        one_hot = torch.zeros_like(output)
        one_hot[0][class_idx] = 1
        output.backward(gradient=one_hot)
        
        # Grad-CAM calculation
        weights = self.gradients.mean(dim=(2, 3), keepdim=True)
        cam = (weights * self.activations).sum(dim=1, keepdim=True)
        cam = torch.relu(cam)
        cam = F.interpolate(cam, x.shape[2:], mode='bilinear', align_corners=False)
        cam = cam - cam.min()
        cam = cam / cam.max()
        return cam.squeeze().cpu().numpy()

# Usage
target_layer = model.features[-3]  # Last conv layer
gradcam = GradCAM(model, target_layer)
cam = gradcam(input_image)
plt.imshow(cam, cmap='jet', alpha=0.5)
plt.imshow(input_image.squeeze().permute(1,2,0), alpha=0.5)
plt.show()

---

## 🔹 Advanced Architectures
### 1. Residual Connections (ResNet)

class ResidualBlock(nn.Module):
    def __init__(self, in_channels, out_channels, stride=1):
        super().__init__()
        self.conv1 = nn.Conv2d(in_channels, out_channels, kernel_size=3, 
                              stride=stride, padding=1, bias=False)
        self.bn1 = nn.BatchNorm2d(out_channels)
        self.conv2 = nn.Conv2d(out_channels, out_channels, kernel_size=3,
                              stride=1, padding=1, bias=False)
        self.bn2 = nn.BatchNorm2d(out_channels)
        
        # Shortcut connection
        self.shortcut = nn.Sequential()
        if stride != 1 or in_channels != out_channels:
            self.shortcut = nn.Sequential(
                nn.Conv2d(in_channels, out_channels, kernel_size=1,
                         stride=stride, bias=False),
                nn.BatchNorm2d(out_channels)
            )
            
    def forward(self, x):
        out = F.relu(self.bn1(self.conv1(x)))
        out = self.bn2(self.conv2(out)))
        out += self.shortcut(x)
        out = F.relu(out)
        return out

### 2. Inception Module

class InceptionModule(nn.Module):
    def __init__(self, in_channels):
        super().__init__()
        
        # 1x1 branch
        self.branch1x1 = nn.Conv2d(in_channels, 64, kernel_size=1)
        
        # 3x3 branch
        self.branch3x3 = nn.Sequential(
            nn.Conv2d(in_channels, 96, kernel_size=1),
            nn.Conv2d(96, 128, kernel_size=3, padding=1)
        )
        
        # 5x5 branch
        self.branch5x5 = nn.Sequential(
            nn.Conv2d(in_channels, 16, kernel_size=1),
            nn.Conv2d(16, 32, kernel_size=5, padding=2)
        )
        
        # Pool branch
        self.branch_pool = nn.Sequential(
            nn.MaxPool2d(kernel_size=3, stride=1, padding=1),
            nn.Conv2d(in_channels, 32, kernel_size=1)
        )
        
    def forward(self, x):
        return torch.cat([
            self.branch1x1(x),
            self.branch3x3(x),
            self.branch5x5(x),
            self.branch_pool(x)
        ], dim=1)

---

# 📚 PyTorch Tutorial for Beginners - Part 4/6: Sequence Modeling with RNNs, LSTMs & Attention
#PyTorch #DeepLearning #NLP #RNN #LSTM #Transformer

Welcome to Part 4 of our PyTorch series! This comprehensive lesson dives deep into sequence modeling, covering recurrent networks, attention mechanisms, and transformer architectures with practical implementations.

---

## 🔹 Introduction to Sequence Modeling
### Key Challenges with Sequences
1. Variable Length: Sequences can be arbitrarily long (sentences, time series)
2. Temporal Dependencies: Current output depends on previous inputs
3. Context Preservation: Need to maintain long-range relationships

### Comparison of Approaches
| Model Type | Pros | Cons | Typical Use Cases |
|------------------|---------------------------------------|---------------------------------------|---------------------------------|
| RNN | Simple, handles sequences | Struggles with long-term dependencies | Short time series, char-level NLP |
| LSTM | Better long-term memory | Computationally heavier | Machine translation, speech recognition |
| GRU | LSTM-like with fewer parameters | Still limited context | Medium-length sequences |
| Transformer | Parallel processing, global context | Memory intensive for long sequences | Modern NLP, any sequence task |

---

## 🔹 Recurrent Neural Networks (RNNs)
### 1. Basic RNN Architecture

class VanillaRNN(nn.Module):
    def __init__(self, input_size, hidden_size, output_size):
        super().__init__()
        self.hidden_size = hidden_size
        self.rnn = nn.RNN(input_size, hidden_size, batch_first=True)
        self.fc = nn.Linear(hidden_size, output_size)
        
    def forward(self, x, hidden=None):
        # x shape: (batch, seq_len, input_size)
        out, hidden = self.rnn(x, hidden)
        # Only use last output for classification
        out = self.fc(out[:, -1, :])  
        return out

# Usage
rnn = VanillaRNN(input_size=10, hidden_size=20, output_size=5)
x = torch.randn(3, 15, 10)  # (batch=3, seq_len=15, input_size=10)
output = rnn(x)

### 2. The Vanishing Gradient Problem
RNNs struggle with long sequences due to:
- Repeated multiplication of small gradients through time
- Exponential decay of gradient information

Solutions:
- Gradient clipping
- Architectural changes (LSTM, GRU)
- Skip connections

---

## 🔹 Long Short-Term Memory (LSTM) Networks
### 1. LSTM Core Concepts

Key Components:
- Forget Gate: Decides what information to discard
- Input Gate: Updates cell state with new information
- Output Gate: Determines next hidden state

### 2. PyTorch Implementation

class LSTMModel(nn.Module):
    def __init__(self, input_size, hidden_size, num_layers, output_size):
        super().__init__()
        self.lstm = nn.LSTM(input_size, hidden_size, num_layers, 
                           batch_first=True, dropout=0.2 if num_layers>1 else 0)
        self.fc = nn.Linear(hidden_size, output_size)
        
    def forward(self, x):
        # Initialize hidden state and cell state
        h0 = torch.zeros(self.lstm.num_layers, x.size(0), 
                        self.lstm.hidden_size).to(x.device)
        c0 = torch.zeros_like(h0)
        
        out, (hn, cn) = self.lstm(x, (h0, c0))
        out = self.fc(out[:, -1, :])
        return out

# Bidirectional LSTM example
bidir_lstm = nn.LSTM(input_size=10, hidden_size=20, num_layers=2,
                    bidirectional=True, batch_first=True)

### 3. Practical Tips for LSTMs
- Initialization: Orthogonal initialization for recurrent weights
- Dropout: Use dropout between LSTM layers (not on last layer)
- Sequence Packing: For variable-length sequences:

from torch.nn.utils.rnn import pack_padded_sequence, pad_packed_sequence

# Assume 'lengths' contains original sequence lengths
packed_input = pack_padded_sequence(x, lengths, batch_first=True, enforce_sorted=False)
packed_output, (hn, cn) = lstm(packed_input)
output, _ = pad_packed_sequence(packed_output, batch_first=True)

---

## 🔹 Gated Recurrent Units (GRUs)
### Simplified Alternative to LSTM

class GRUModel(nn.Module):
    def __init__(self, input_size, hidden_size, num_layers, output_size):
        super().__init__()
        self.gru = nn.GRU(input_size, hidden_size, num_layers,
                         batch_first=True, dropout=0.2)
        self.fc = nn.Linear(hidden_size, output_size)
        
    def forward(self, x):
        out, hn = self.gru(x)
        out = self.fc(out[:, -1, :])
        return out

GRU vs LSTM:
- GRU combines forget and input gates into update gate
- GRU merges cell state and hidden state
- Typically faster to train with comparable performance

---

## 🔹 Attention Mechanisms
### 1. Why Attention?
- Addresses information bottleneck in encoder-decoder architectures
- Allows dynamic focus on relevant parts of input
- Enables interpretability (visualize attention weights)

### 2. Basic Attention Implementation

class Attention(nn.Module):
    def __init__(self, hidden_size):
        super().__init__()
        self.attention = nn.Linear(hidden_size, hidden_size)
        self.context = nn.Parameter(torch.randn(hidden_size))
        
    def forward(self, hidden_states):
        # hidden_states shape: (batch, seq_len, hidden_size)
        
        # Compute attention scores
        energies = torch.tanh(self.attention(hidden_states))
        scores = torch.matmul(energies, self.context)
        alphas = torch.softmax(scores, dim=1)
        
        # Compute context vector
        context = torch.sum(hidden_states * alphas.unsqueeze(-1), dim=1)
        return context, alphas

# Integrated with RNN
class AttnRNN(nn.Module):
    def __init__(self, input_size, hidden_size, output_size):
        super().__init__()
        self.rnn = nn.GRU(input_size, hidden_size, batch_first=True)
        self.attention = Attention(hidden_size)
        self.fc = nn.Linear(hidden_size, output_size)
        
    def forward(self, x):
        out, _ = self.rnn(x)
        context, alphas = self.attention(out)
        return self.fc(context), alphas

### 3. Visualizing Attention

def plot_attention(input_text, alphas):
    fig, ax = plt.subplots(figsize=(12, 4))
    ax.imshow(alphas.cpu().numpy(), cmap='viridis')
    ax.set_xticks(range(len(input_text.split())))
    ax.set_xticklabels(input_text.split(), rotation=45)
    ax.set_yticks([0])
    ax.set_yticklabels(['Attention'])
    plt.tight_layout()
    plt.show()

---

## 🔹 Transformer Architectures
### 1. Core Components

Key Innovations:
- Self-Attention: Captures relationships between all sequence positions
- Positional Encoding: Injects sequence order information
- Layer Normalization: Stabilizes training
- Feed-Forward Networks: Applied position-wise

### 2. Implementing Multi-Head Attention

class MultiHeadAttention(nn.Module):
    def __init__(self, embed_size, heads):
        super().__init__()
        self.embed_size = embed_size
        self.heads = heads
        self.head_dim = embed_size // heads
        
        assert self.head_dim * heads == embed_size, "Embed size needs to be divisible by heads"
        
        self.values = nn.Linear(self.head_dim, self.head_dim, bias=False)
        self.keys = nn.Linear(self.head_dim, self.head_dim, bias=False)
        self.queries = nn.Linear(self.head_dim, self.head_dim, bias=False)
        self.fc_out = nn.Linear(heads * self.head_dim, embed_size)
        
    def forward(self, values, keys, query, mask=None):
        N = query.shape[0]
        value_len, key_len, query_len = values.shape[1], keys.shape[1], query.shape[1]
        
        # Split embedding into self.heads pieces
        values = values.reshape(N, value_len, self.heads, self.head_dim)
        keys = keys.reshape(N, key_len, self.heads, self.head_dim)
        queries = query.reshape(N, query_len, self.heads, self.head_dim)
        
        # Attention scores
        energy = torch.einsum("nqhd,nkhd->nhqk", [queries, keys])
        
        if mask is not None:
            energy = energy.masked_fill(mask == 0, float("-1e20"))
            
        attention = torch.softmax(energy / (self.embed_size ** (1/2)), dim=3)
        
        out = torch.einsum("nhql,nlhd->nqhd", [attention, values]).reshape(
            N, query_len, self.heads * self.head_dim
        )
        
        out = self.fc_out(out)
        return out

### **3. Complete Transformer Block**

class TransformerBlock(nn.Module):
    def __init__(self, embed_size, heads, dropout, forward_expansion):
        super().__init__()
        self.attention = MultiHeadAttention(embed_size, heads)
        self.norm1 = nn.LayerNorm(embed_size)
        self.norm2 = nn.LayerNorm(embed_size)
        
        self.feed_forward = nn.Sequential(
            nn.Linear(embed_size, forward_expansion * embed_size),
            nn.ReLU(),
            nn.Linear(forward_expansion * embed_size, embed_size)
        )
        
        self.dropout = nn.Dropout(dropout)
        
    def forward(self, value, key, query, mask):
        attention = self.attention(value, key, query, mask)
        x = self.dropout(self.norm1(attention + query))
        forward = self.feed_forward(x)
        out = self.dropout(self.norm2(forward + x))
        return out

### 4. Positional Encoding

class PositionalEncoding(nn.Module):
    def __init__(self, embed_size, max_len=100):
        super().__init__()
        pe = torch.zeros(max_len, embed_size)
        position = torch.arange(0, max_len, dtype=torch.float).unsqueeze(1)
        div_term = torch.exp(torch.arange(0, embed_size, 2).float() * (-math.log(10000.0) / embed_size)
        
        pe[:, 0::2] = torch.sin(position * div_term)
        pe[:, 1::2] = torch.cos(position * div_term)
        pe = pe.unsqueeze(0)
        self.register_buffer('pe', pe)
        
    def forward(self, x):
        return x + self.pe[:, :x.size(1)]

---

## 🔹 Practical Sequence Modeling Tasks
### 1. Text Classification Pipeline

from torchtext.legacy import data

# Define fields
TEXT = data.Field(tokenize='spacy', lower=True, include_lengths=True)
LABEL = data.LabelField(dtype=torch.float)

# Load dataset (e.g., IMDB)
train_data, test_data = datasets.IMDB.splits(TEXT, LABEL)

# Build vocabulary
TEXT.build_vocab(train_data, max_size=25000, 
                vectors="glove.6B.100d", unk_init=torch.Tensor.normal_)
LABEL.build_vocab(train_data)

# Create iterators
train_loader, test_loader = data.BucketIterator.splits(
    (train_data, test_data), 
    batch_size=64,
    sort_within_batch=True,
    sort_key=lambda x: len(x.text),
    device=device
)

# Model definition
class TextClassifier(nn.Module):
    def __init__(self, vocab_size, embed_dim, hidden_dim, output_dim, n_layers):
        super().__init__()
        self.embedding = nn.Embedding(vocab_size, embed_dim)
        self.rnn = nn.LSTM(embed_dim, hidden_dim, n_layers, 
                          bidirectional=True, dropout=0.5)
        self.fc = nn.Linear(hidden_dim * 2, output_dim)
        self.dropout = nn.Dropout(0.5)
        
    def forward(self, text, text_lengths):
        embedded = self.dropout(self.embedding(text))
        packed_embedded = nn.utils.rnn.pack_padded_sequence(
            embedded, text_lengths.cpu(), batch_first=False, enforce_sorted=False
        )
        packed_output, (hidden, cell) = self.rnn(packed_embedded)
        hidden = self.dropout(torch.cat((hidden[-2,:,:], hidden[-1,:,:]), dim=1))
        return self.fc(hidden)

### 2. Sequence-to-Sequence (Seq2Seq) Model

class Encoder(nn.Module):
    def __init__(self, input_dim, emb_dim, hidden_dim, n_layers, dropout):
        super().__init__()
        self.embedding = nn.Embedding(input_dim, emb_dim)
        self.rnn = nn.LSTM(emb_dim, hidden_dim, n_layers, dropout=dropout)
        self.dropout = nn.Dropout(dropout)
        
    def forward(self, src):
        embedded = self.dropout(self.embedding(src))
        outputs, (hidden, cell) = self.rnn(embedded)
        return hidden, cell

class Decoder(nn.Module):
    def __init__(self, output_dim, emb_dim, hidden_dim, n_layers, dropout):
        super().__init__()
        self.embedding = nn.Embedding(output_dim, emb_dim)
        self.rnn = nn.LSTM(emb_dim, hidden_dim, n_layers, dropout=dropout)
        self.fc = nn.Linear(hidden_dim, output_dim)
        self.dropout = nn.Dropout(dropout)
        
    def forward(self, input, hidden, cell):
        input = input.unsqueeze(0)
        embedded = self.dropout(self.embedding(input))
        output, (hidden, cell) = self.rnn(embedded, (hidden, cell))
        prediction = self.fc(output.squeeze(0))
        return prediction, hidden, cell

class Seq2Seq(nn.Module):
    def __init__(self, encoder, decoder, device):
        super().__init__()
        self.encoder = encoder
        self.decoder = decoder
        self.device = device
        
    def forward(self, src, trg, teacher_forcing_ratio=0.5):
        trg_len = trg.shape[0]
        batch_size = trg.shape[1]
        trg_vocab_size = self.decoder.fc.out_features
        
        outputs = torch.zeros(trg_len, batch_size, trg_vocab_size).to(self.device)
        hidden, cell = self.encoder(src)
        
        input = trg[0,:]
        
        for t in range(1, trg_len):
            output, hidden, cell = self.decoder(input, hidden, cell)
            outputs[t] = output
            teacher_force = random.random() < teacher_forcing_ratio
            top1 = output.argmax(1)
            input = trg[t] if teacher_force else top1
            
        return outputs

---

## 🔹 Best Practices for Sequence Modeling
1. Always use packed sequences for variable-length inputs
2. Gradient clipping is essential for RNNs/LSTMs (1-5 norm)
3. Teacher forcing helps seq2seq models converge faster
4. Bidirectional RNNs significantly improve performance
5. Layer normalization stabilizes transformer training
6. Warmup learning rate for transformer models

# Learning rate scheduler for transformers
def lr_schedule(step, d_model=512, warmup_steps=4000):
    arg1 = step ** -0.5
    arg2 = step * (warmup_steps ** -1.5)
    return (d_model ** -0.5) * min(step ** -0.5, step * warmup_steps ** -1.5)

---

### **📌 What's Next?
In **Part 5, we'll cover:
➡️ Generative Models (GANs, VAEs)
➡️ Reinforcement Learning with PyTorch
➡️ Model Optimization & Deployment
➡️ PyTorch Lightning Best Practices

#PyTorch #DeepLearning #NLP #Transformers 🚀

Practice Exercises:
1. Implement a character-level language model with LSTM
2. Add attention visualization to a sentiment analysis model
3. Build a transformer from scratch for machine translation
4. Compare teacher forcing ratios in seq2seq training
5. Implement beam search for decoder inference

# Character-level LSTM starter
class CharLSTM(nn.Module):
    def __init__(self, vocab_size, hidden_size, n_layers):
        super().__init__()
        self.embed = nn.Embedding(vocab_size, hidden_size)
        self.lstm = nn.LSTM(hidden_size, hidden_size, n_layers, batch_first=True)
        self.fc = nn.Linear(hidden_size, vocab_size)
        
    def forward(self, x, hidden=None):
        x = self.embed(x)
        out, hidden = self.lstm(x, hidden)
        return self.fc(out), hidden

# 📚 PyTorch Tutorial for Beginners - Part 5/6: Generative Models & Advanced Topics
#PyTorch #DeepLearning #GANs #VAEs #ReinforcementLearning #Deployment

Welcome to Part 5 of our PyTorch series! This comprehensive lesson explores generative modeling, reinforcement learning, model optimization, and deployment strategies with practical implementations.

---

## 🔹 Generative Adversarial Networks (GANs)
### 1. GAN Core Concepts

Key Components:
- Generator: Creates fake samples from noise (typically a transposed CNN)
- Discriminator: Distinguishes real vs. fake samples (CNN classifier)
- Adversarial Training: The two networks compete in a minimax game

### 2. DCGAN Implementation

class Generator(nn.Module):
    def __init__(self, latent_dim, img_channels, features_g):
        super().__init__()
        self.net = nn.Sequential(
            # Input: N x latent_dim x 1 x 1
            nn.ConvTranspose2d(latent_dim, features_g*8, 4, 1, 0, bias=False),
            nn.BatchNorm2d(features_g*8),
            nn.ReLU(),
            # 4x4
            nn.ConvTranspose2d(features_g*8, features_g*4, 4, 2, 1, bias=False),
            nn.BatchNorm2d(features_g*4),
            nn.ReLU(),
            # 8x8
            nn.ConvTranspose2d(features_g*4, features_g*2, 4, 2, 1, bias=False),
            nn.BatchNorm2d(features_g*2),
            nn.ReLU(),
            # 16x16
            nn.ConvTranspose2d(features_g*2, img_channels, 4, 2, 1, bias=False),
            nn.Tanh()
            # 32x32
        )

    def forward(self, x):
        return self.net(x)

class Discriminator(nn.Module):
    def __init__(self, img_channels, features_d):
        super().__init__()
        self.net = nn.Sequential(
            # Input: N x img_channels x 32 x 32
            nn.Conv2d(img_channels, features_d, 4, 2, 1, bias=False),
            nn.LeakyReLU(0.2),
            # 16x16
            nn.Conv2d(features_d, features_d*2, 4, 2, 1, bias=False),
            nn.BatchNorm2d(features_d*2),
            nn.LeakyReLU(0.2),
            # 8x8
            nn.Conv2d(features_d*2, features_d*4, 4, 2, 1, bias=False),
            nn.BatchNorm2d(features_d*4),
            nn.LeakyReLU(0.2),
            # 4x4
            nn.Conv2d(features_d*4, 1, 4, 1, 0, bias=False),
            nn.Sigmoid()
        )

    def forward(self, x):
        return self.net(x)

# Initialize
gen = Generator(latent_dim=100, img_channels=3, features_g=64).to(device)
disc = Discriminator(img_channels=3, features_d=64).to(device)

# Loss and optimizers
criterion = nn.BCELoss()
opt_gen = optim.Adam(gen.parameters(), lr=0.0002, betas=(0.5, 0.999))
opt_disc = optim.Adam(disc.parameters(), lr=0.0002, betas=(0.5, 0.999))

### 3. GAN Training Loop

def train_gan(gen, disc, loader, num_epochs):
    fixed_noise = torch.randn(32, 100, 1, 1).to(device)
    
    for epoch in range(num_epochs):
        for batch_idx, (real, _) in enumerate(loader):
            real = real.to(device)
            noise = torch.randn(real.size(0), 100, 1, 1).to(device)
            fake = gen(noise)
            
            # Train Discriminator
            disc_real = disc(real).view(-1)
            loss_disc_real = criterion(disc_real, torch.ones_like(disc_real))
            disc_fake = disc(fake.detach()).view(-1)
            loss_disc_fake = criterion(disc_fake, torch.zeros_like(disc_fake))
            loss_disc = (loss_disc_real + loss_disc_fake) / 2
            disc.zero_grad()
            loss_disc.backward()
            opt_disc.step()
            
            # Train Generator
            output = disc(fake).view(-1)
            loss_gen = criterion(output, torch.ones_like(output))
            gen.zero_grad()
            loss_gen.backward()
            opt_gen.step()
            
        # Visualization
        with torch.no_grad():
            fake = gen(fixed_noise)
            save_image(fake, f"gan_samples/epoch_{epoch}.png", normalize=True)

### 4. GAN Training Challenges & Solutions
| Problem | Solution | Implementation Tips |
|-----------------------|-----------------------------------|---------------------------------------|
| Mode Collapse | Mini-batch discrimination | Use torch.cat for batch statistics |
| Vanishing Gradients | Wasserstein GAN with GP | Clip critic weights or use gradient penalty |
| Unstable Training | Two Time-Scale Update Rule (TTUR) | Different learning rates for G/D |
| Poor Image Quality | Spectral Normalization | torch.nn.utils.spectral_norm layers |

---

## 🔹 Variational Autoencoders (VAEs)
### 1. VAE Core Concepts

Key Components:
- Encoder: Maps input to latent space distribution parameters (μ, σ)
- Reparameterization Trick: Allows backpropagation through sampling
- Decoder: Reconstructs input from latent samples

### 2. VAE Implementation

class VAE(nn.Module):
    def __init__(self, input_dim, hidden_dim, latent_dim):
        super().__init__()
        
        # Encoder
        self.encoder = nn.Sequential(
            nn.Linear(input_dim, hidden_dim),
            nn.ReLU(),
            nn.Linear(hidden_dim, hidden_dim),
            nn.ReLU()
        )
        
        # Latent space parameters
        self.fc_mu = nn.Linear(hidden_dim, latent_dim)
        self.fc_var = nn.Linear(hidden_dim, latent_dim)
        
        # Decoder
        self.decoder = nn.Sequential(
            nn.Linear(latent_dim, hidden_dim),
            nn.ReLU(),
            nn.Linear(hidden_dim, hidden_dim),
            nn.ReLU(),
            nn.Linear(hidden_dim, input_dim),
            nn.Sigmoid()
        )
    
    def encode(self, x):
        h = self.encoder(x)
        return self.fc_mu(h), self.fc_var(h)
    
    def reparameterize(self, mu, logvar):
        std = torch.exp(0.5 * logvar)
        eps = torch.randn_like(std)
        return mu + eps * std
    
    def decode(self, z):
        return self.decoder(z)
    
    def forward(self, x):
        mu, logvar = self.encode(x)
        z = self.reparameterize(mu, logvar)
        return self.decode(z), mu, logvar

# Loss function
def vae_loss(recon_x, x, mu, logvar):
    BCE = nn.functional.binary_cross_entropy(recon_x, x, reduction='sum')
    KLD = -0.5 * torch.sum(1 + logvar - mu.pow(2) - logvar.exp())
    return BCE + KLD

### 3. Conditional VAE (CVAE)

class CVAE(nn.Module):
    def __init__(self, input_dim, hidden_dim, latent_dim, num_classes):
        super().__init__()
        self.label_emb = nn.Embedding(num_classes, num_classes)
        
        # Encoder now takes both image and label
        self.encoder = nn.Sequential(
            nn.Linear(input_dim + num_classes, hidden_dim),
            nn.ReLU()
        )
        
        # Rest remains similar to VAE...

---

## 🔹 Reinforcement Learning with PyTorch
### 1. Deep Q-Network (DQN)

class DQN(nn.Module):
    def __init__(self, input_dim, output_dim):
        super().__init__()
        self.fc = nn.Sequential(
            nn.Linear(input_dim, 128),
            nn.ReLU(),
            nn.Linear(128, 128),
            nn.ReLU(),
            nn.Linear(128, output_dim)
        )
    
    def forward(self, x):
        return self.fc(x)

# Experience Replay
class ReplayBuffer:
    def __init__(self, capacity):
        self.buffer = deque(maxlen=capacity)
    
    def push(self, state, action, reward, next_state, done):
        self.buffer.append((state, action, reward, next_state, done))
    
    def sample(self, batch_size):
        return random.sample(self.buffer, batch_size)

# Training loop
def train_dqn(env, model, target_model, optimizer, buffer, 
             batch_size=64, gamma=0.99):
    if len(buffer) < batch_size:
        return
    
    transitions = buffer.sample(batch_size)
    batch = list(zip(*transitions))
    
    states = torch.FloatTensor(np.array(batch[0])).to(device)
    actions = torch.LongTensor(batch[1]).unsqueeze(1).to(device)
    rewards = torch.FloatTensor(batch[2]).unsqueeze(1).to(device)
    next_states = torch.FloatTensor(np.array(batch[3])).to(device)
    dones = torch.FloatTensor(batch[4]).unsqueeze(1).to(device)
    
    current_q = model(states).gather(1, actions)
    next_q = target_model(next_states).max(1)[0].detach().unsqueeze(1)
    target_q = rewards + (gamma * next_q * (1 - dones))
    
    loss = nn.functional.mse_loss(current_q, target_q)
    optimizer.zero_grad()
    loss.backward()
    optimizer.step()

### 2. Policy Gradient Methods

class PolicyNetwork(nn.Module):
    def __init__(self, input_dim, output_dim):
        super().__init__()
        self.fc = nn.Sequential(
            nn.Linear(input_dim, 128),
            nn.ReLU(),
            nn.Linear(128, output_dim),
            nn.Softmax(dim=-1)
        )
    
    def forward(self, x):
        return self.fc(x)

def train_policy_gradient(env, model, optimizer, num_episodes):
    for episode in range(num_episodes):
        state = env.reset()
        log_probs = []
        rewards = []
        
        while True:
            state = torch.FloatTensor(state).unsqueeze(0).to(device)
            probs = model(state)
            action_dist = torch.distributions.Categorical(probs)
            action = action_dist.sample()
            
            next_state, reward, done, _ = env.step(action.item())
            
            log_probs.append(action_dist.log_prob(action))
            rewards.append(reward)
            state = next_state
            
            if done:
                break
                
        # Calculate discounted rewards
        discounted_rewards = []
        R = 0
        for r in reversed(rewards):
            R = r + gamma * R
            discounted_rewards.insert(0, R)
        
        # Normalize rewards
        discounted_rewards = torch.FloatTensor(discounted_rewards).to(device)
        discounted_rewards = (discounted_rewards - discounted_rewards.mean()) / \
                           (discounted_rewards.std() + 1e-9)
        
        # Calculate loss
        policy_loss = []
        for log_prob, reward in zip(log_probs, discounted_rewards):
            policy_loss.append(-log_prob * reward)
        
        optimizer.zero_grad()
        policy_loss = torch.cat(policy_loss).sum()
        policy_loss.backward()
        optimizer.step()

---

## 🔹 Model Optimization & Deployment
### 1. Quantization

# Dynamic quantization
model = nn.Sequential(
    nn.Linear(64, 128),
    nn.ReLU(),
    nn.Linear(128, 10)
)
quantized_model = torch.quantization.quantize_dynamic(
    model, {nn.Linear}, dtype=torch.qint8
)

# Post-training static quantization
model.qconfig = torch.quantization.get_default_qconfig('fbgemm')
torch.quantization.prepare(model, inplace=True)
# Calibrate with sample data
torch.quantization.convert(model, inplace=True)