If you change the model definition, the pretrained state_dict cannot be loaded and you’ll get the error message:
RuntimeError: Error(s) in loading state_dict for ResNet:
	size mismatch for conv1.weight: copying a param with shape torch.Size([64, 3, 7, 7]) from checkpoint, the shape in current model is torch.Size([64, 4, 7, 7]).

I assume either your resnet definition wasn’t updated or you didn’t use the pretrained weights.