MarianMT int dtype fix
Found the same integer dtype issue in MarianMT as in T5 - I'll probably need to find a proper way of dealing with this across the codebase at some point, but this'll do to get the notebook examples working at least.
Update: I found multiple Seq2Seq models were copying the problematic `shift_tokens_right` method from BART, so I fixed the BART method and ran `make fix-copies`. That should hopefully fix this issue for a lot of our models.