Bug fix for permutation language modelling
Addresses a bug in permutation language modelling data collator, where `&` is used instead of `|` to compute the non-functional token mask (tokens excluding [PAD], [SEP], [CLS]). For verification, may refer to original XLNet code (https://github.com/zihangdai/xlnet/blob/master/data_utils.py#L602).

Addresses #6812 (further investigation needed, however)
 
@patrickvonplaten @LysandreJik 