[BUG][Tabular] bad validation on model classes

[Nadav-Barak]

We should validate the the model classes contain all observed labels + is of the same length of the proba

Currently it is done partially and spraded in different places in the package.

Suggestion:

In context add functions of get_predictions() and get_probabilities() and use them instead of calling the context.model.predict_proba functionality. Validation should be done on the first time those functions are called