src/models/resnet18_finetune.py

import torch.nn as nn
from torchvision import models

def make_resnet18(num_classes=39):
    """
    Constructs a ResNet18 model with a custom classification head for PlantVillage.
    The feature extractor weights are frozen (transfer learning).
    """
    # Load pretrained model
    # Note: In the lab we used weights=...V1. Here we use the same.
    model = models.resnet18(weights=models.ResNet18_Weights.IMAGENET1K_V1)
    
    # Freeze feature extractor
    for param in model.parameters():
        param.requires_grad = False

    num_ftrs = model.fc.in_features
    model.fc = nn.Linear(num_ftrs, num_classes)
    
    return model