Clarify requirements for ref_labels, or change get_all_pairs_indices and get_all_triplets_indices

[KevinMusgrave]

It's normal to try:

labels = torch.arange(32)
x = lmu.get_all_pairs_indices(labels, ref_labels=labels)

This will result in 0 positive pairs because of the labels is ref_labels check. This isn't documented anywhere though.

See #495