training/models/efficientnet_b4.py

# training/models/efficientnet_b4.py
"""EfficientNet-B4 dual-head model for hemoglobin regression + anemia classification."""

import timm
import torch
import torch.nn as nn


class AnemiaModel(nn.Module):
    """
    EfficientNet-B4 backbone with dual prediction heads:
      - Regression head: predicts Hb (g/dL)
      - Classification head: predicts 4-class anemia severity
    """

    def __init__(
        self,
        num_classes: int = 4,
        dropout_rate: float = 0.3,
        pretrained: bool = True,
    ):
        super().__init__()
        self.backbone = timm.create_model(
            "efficientnet_b4",
            pretrained=pretrained,
            num_classes=0,  # remove classifier head
            global_pool="avg",
        )
        feature_dim = self.backbone.num_features

        self.regression_head = nn.Sequential(
            nn.Linear(feature_dim, 256),
            nn.ReLU(),
            nn.Dropout(dropout_rate),
            nn.Linear(256, 1),
        )
        self.classification_head = nn.Sequential(
            nn.Linear(feature_dim, 256),
            nn.ReLU(),
            nn.Dropout(dropout_rate),
            nn.Linear(256, num_classes),
        )

    def forward(self, x: torch.Tensor) -> tuple[torch.Tensor, torch.Tensor]:
        features = self.backbone(x)
        hb_pred = self.regression_head(features)
        class_logits = self.classification_head(features)
        return hb_pred, class_logits

    def freeze_backbone(self):
        for param in self.backbone.parameters():
            param.requires_grad = False

    def unfreeze_last_n_blocks(self, n: int = 3):
        """Unfreeze last n blocks of the backbone for fine-tuning."""
        blocks = list(self.backbone.blocks)
        for block in blocks[-n:]:
            for param in block.parameters():
                param.requires_grad = True
        # Always unfreeze the final conv + bn
        for param in self.backbone.conv_head.parameters():
            param.requires_grad = True
        for param in self.backbone.bn2.parameters():
            param.requires_grad = True