src/model.py

# src/model.py
import torch
import torch.nn as nn
import torchvision.models as models
import torch.nn.functional as F

class EmbeddingNet(nn.Module):
    def __init__(self, embedding_dim=128):
        super().__init__()

        # Pretrained ResNet-18, strip the final FC layer
        backbone = models.resnet18(weights=models.ResNet18_Weights.DEFAULT)
        self.backbone = nn.Sequential(*list(backbone.children())[:-1])  # → [B, 512, 1, 1]

        # Embedding head: 512 → 256 → 128, L2-normalised output
        self.head = nn.Sequential(
            nn.Linear(512, 256),
            nn.BatchNorm1d(256),
            nn.ReLU(inplace=True),
            nn.Linear(256, embedding_dim),
        )

    def forward(self, x):
        # Omniglot is grayscale — replicate channel to fake RGB for ResNet
        if x.shape[1] == 1:
            x = x.repeat(1, 3, 1, 1)               # [B, 1, H, W] → [B, 3, H, W]

        x = self.backbone(x)                        # [B, 512, 1, 1]
        x = x.view(x.size(0), -1)                  # [B, 512]
        x = self.head(x)                            # [B, 128]
        x = F.normalize(x, p=2, dim=1)             # L2 normalise → unit sphere
        return x


class SiameseNet(nn.Module):
    def __init__(self, embedding_dim=128):
        super().__init__()
        self.embedding_net = EmbeddingNet(embedding_dim)

    def forward(self, img1, img2):
        emb1 = self.embedding_net(img1)
        emb2 = self.embedding_net(img2)
        return emb1, emb2

    def get_embedding(self, img):
        return self.embedding_net(img)