model.py

import torch
import torch.nn as nn

device = "cuda:0" if torch.cuda.is_available() else "cpu"


class Linear(nn.Module):
    def __init__(self, in_features: int, out_features: int, std: float = 0.1):
        """
        Initialize the linear layer with random values for weights.

        The weights and biases are registered as parameters, allowing for
        gradient computation and update during backpropagation.
        """
        super(Linear, self).__init__()
        self.in_features = in_features
        self.out_features = out_features

        weight = torch.randn(in_features, out_features, requires_grad=True) * std
        bias = torch.zeros(out_features, requires_grad=True) * std

        self.weight = nn.Parameter(weight)
        self.bias = nn.Parameter(bias)

        self.to(device=device)

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        """
        Perform linear transformation by multiplying the input tensor
        with the weight matrix, and adding the bias.
        """
        return x @ self.weight + self.bias

    def __repr__(self) -> str:
        return f"in_features={self.in_features}, out_features={self.out_features}, bias={self.bias is not None}"


class ReLU(nn.Module):
    """
    Rectified Linear Unit (ReLU) activation function.
    """

    @staticmethod
    def forward(x: torch.Tensor) -> torch.Tensor:
        return torch.max(x, torch.zeros_like(x))


class Sequential(nn.Module):
    """
    Sequential container for stacking multiple modules,
    passing the output of one module as input to the next.
    """

    def __init__(self, *layers):
        """
        Initialize the Sequential container with a list of layers.
        """
        super(Sequential, self).__init__()
        self.layers = nn.ModuleList(layers)

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        for layer in self.layers:
            x = layer(x)
        return x

    def __repr__(self) -> str:
        layer_str = "\n".join(
            [f" ({i}): {layer}" for i, layer in enumerate(self.layers)]
        )
        return f"{self.__class__.__name__}(\n{layer_str}\n)"


class Flatten(nn.Module):
    """
    Reshape the input tensor by flattening all dimensions except the first dimension.
    """

    @staticmethod
    def forward(x: torch.Tensor) -> torch.Tensor:
        """
        Note that x.view(x.size(0), -1) reshapes the x tensor to (x.size(0), N)
        where N is the product of the remaining dimensions.

        E.g. (batch_size, 28, 28) -> (batch_size, 784)
        """
        return x.view(x.size(0), -1)


class Dropout(nn.Module):
    """
    Dropout layer for regularization by randomly setting input elements to zero
    with probability p during training.
    """

    def __init__(self, p=0.2):
        super(Dropout, self).__init__()
        self.p = p

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        if self.training:
            mask = (torch.rand(x.shape) > self.p).float().to(x) / (1 - self.p)
            return x * mask
        return x


class Classifier(nn.Module):
    """
    Classifier model consisting of a sequence of linear layers and ReLU activations,
    followed by a final linear layer that outputs logits (unnormalized scores)
    for each of the 10 garment classes.
    """

    def __init__(self):
        """
        The output logits of the last layer can be passed directly to
        a loss function like CrossEntropyLoss, which will apply the
        softmax function internally to calculate a probability distribution.
        """
        super(Classifier, self).__init__()
        self.labels = [
            "T-shirt/Top",
            "Trouser/Jeans",
            "Pullover",
            "Dress",
            "Coat",
            "Sandal",
            "Shirt",
            "Sneaker",
            "Bag",
            "Ankle-Boot",
        ]

        self.main = Sequential(
            Flatten(),
            Linear(in_features=784, out_features=256),
            ReLU(),
            Dropout(0.2),
            Linear(in_features=256, out_features=64),
            ReLU(),
            Dropout(0.2),
            Linear(in_features=64, out_features=10),
        )

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        return self.main(x)

    def predictions(self, x):
        with torch.no_grad():
            logits = self.forward(x)
            probs = torch.nn.functional.softmax(logits, dim=1)
            predictions = dict(zip(self.labels, probs.cpu().detach().numpy().flatten()))
        return predictions