loading saved model when n_classes != 2
Required to for: Assertion `t >= 0 && t < n_classes` failed,  if your default number of classes is not 2.
This problem is discussed is #135 and I don't think that this is the right way to patch this problem. The saved model contains the `num_labels` information.<|||||>cf discussion in #135 let's go for the `mandatory-argument`solution for now.