I assume you are using a higher-level API, which provides this fit_sm method.
If so, it should internally execute the forward and backward passes, as well as the optimization via:
optimizer.zero_grad()
output = model(data)
loss = criterion(output, target)
loss.backward()
optimizer.step()

I don’t know what else is performed in this method, but these parts should be there at least.
Assuming you’ve floowed the suggestions and added the classifier to the entire model, you should be able to use it directly.
Note that I don’t see how the data is passed to this fit_sm method, but I guess that it might be part of the fp object.