Ok I’ve modified the code as follows, to only update the final FC layer.
ConvNet as fixed feature extractor

model_conv = torchvision.models.alexnet(pretrained=True)
for param in model_conv.parameters():
param.requires_grad = False
num_ftrs = model_conv.classifier[6].in_features
model_conv.classifier[6].out_features = Output_features
for param in model_conv.classifier[6].parameters():
param.requires_grad = True
if use_gpu:
model_conv = model_conv.cuda()
criterion = nn.CrossEntropyLoss()