Bart: make decoder_input_ids correctly if labels specified.
should call shift_tokens_right(labels), like T5.

cc @patil-suraj: does that make sense?
Yes, definitely. Better to handle it in the model. Since lots of recent issues were due to incorrect or not shifting labels<|||||>But then `prepare_seq2seq_batch` should return `labels` instead of `decoder_input_ids`<|||||>yep. I'll take a stab.