3 is
['logits', 'past_key_values', 'encoder_last_hidden_state']. This is what the seq2seq models return. logits is what you’ll need for computing metrics which is of the shape (bs, seq_len, vocab_size).
Also, for training seq2seq models consider using Seq2SeqTrainer 6, which supports generation during evaluation to be able to calculate generative metrics like blue, rouge etc.
Check  finetune_trainer.py 4  to see how to use Seq2SeqTrainer