Exceptions when training with less negative than positive samples / question about `__getitem__`

[AnikaP]

I noticed that there will be exceptions if a sample contains less negative then positive samples. The issues occur in dataset.py, __getitem__. Ensuring the random choice sample size is not too big (size=min(self.num_neg, self.num_negatives_per_one_positive * self.num_pos), dataset.py, line 39) and using num_neg for the mod (ix = index % self.num_neg , dataset.py, line 48) would fix this.