model.py

import torch
import torch.nn as nn
import torchvision.transforms as T


class ConvBlock(nn.Module):
    def __init__(self, in_channels: int, out_channels: int):
        super(ConvBlock, self).__init__()
        self.block = nn.Sequential(
            nn.Conv2d(in_channels, out_channels, kernel_size=3, stride=1, padding=1),
            nn.BatchNorm2d(out_channels),
            nn.ReLU(inplace=True),
            
            nn.Conv2d(out_channels, out_channels, kernel_size=3, stride=1, padding=1),
            nn.BatchNorm2d(out_channels),
            nn.ReLU(inplace=True)
        )
        
    def forward(self, x: torch.Tensor):
        return self.block(x)


class CopyAndCrop(nn.Module):
    def forward(self, x: torch.Tensor, encoded: torch.Tensor):
        _, _, h, w = encoded.shape
        crop = T.CenterCrop((h, w))(x)
        output = torch.cat((x, crop), 1)
        
        return output


class UNet(nn.Module):
    def __init__(self, in_channels: int, out_channels: int):
        super(UNet, self).__init__()

        self.encoders = nn.ModuleList([
            ConvBlock(in_channels, 64),
            ConvBlock(64, 128),
            ConvBlock(128, 256),
            ConvBlock(256, 512),
        ])
        self.down_sample = nn.MaxPool2d(2)
        self.copyAndCrop = CopyAndCrop()
        self.decoders = nn.ModuleList([
            ConvBlock(1024, 512),
            ConvBlock(512, 256),
            ConvBlock(256, 128),
            ConvBlock(128, 64),
        ])

        self.up_samples = nn.ModuleList([
            nn.ConvTranspose2d(1024, 512, kernel_size=2, stride=2),
            nn.ConvTranspose2d(512, 256, kernel_size=2, stride=2),
            nn.ConvTranspose2d(256, 128, kernel_size=2, stride=2),
            nn.ConvTranspose2d(128, 64, kernel_size=2, stride=2)
        ])

        self.bottleneck = ConvBlock(512, 1024)
        self.final_conv = nn.Conv2d(64, out_channels, kernel_size=1, stride=1)
    
    def forward(self, x: torch.Tensor):
        # encode
        encoded_features = []
        for enc in self.encoders:
            x = enc(x)
            encoded_features.append(x)
            x = self.down_sample(x)
            
        x = self.bottleneck(x)

        # decode
        for idx, denc in enumerate(self.decoders):
            x = self.up_samples[idx](x)
            encoded = encoded_features.pop()
            x = self.copyAndCrop(x, encoded)
            x = denc(x)
            
        output = self.final_conv(x)
        return output