Text classification label added.
@VahidooX to discuss:
1. should we make class_labels_file mandatory? and it so, then:
2. could we get rid of num_classes and get it as len(labels)  when loaded from class_labels_file?

The PR looks good and we can absorb it to TLT->JARVIS the way it is , but I am just thinking loud here... 2nd set of thoughts:

1) in fine-tuning we are effectively changing the dataset, hence should change the "class labels" too. We are using the `setup_*_ds`  for that purpose. Do you think those methods should re-load/overwrite those sections in model?
2) should we leverage that information in inference (`classifytext`) ?

(in other words - should we move that part of "post-processing" from TLT inference to `model.classifytext`?)