improve Dataset class, avoiding all the 'if condition: continue' and allow deterministic idx

[laurentbenaroya]

In the file train_vocalist_lrs2.py, in the __getitem__ method of the Dataset class, an idx index was drawn at random, even though idx was supplied as input to the method. As a result, a while True is required and a lot of conditions that lead to a continue if they are not met. I replaced all this with assert not condition, which allowed me to debug the Dataset class.
This has been done in train_vocalist_lrs2.py and could be extended to train_vocalist_acappella.py (if the PR is accepted).
Note small changes in train and eval_model functions (cosmetic).