[seq2seq] correctly handle mt5
This PR fixes `seq2seq/utils.py` to handle `mt5` like it does `t5`.

Ideally there should be a test, which would require creating a tiny model for mt5, but I'm being told this code is going away anyway, so there is no point investing energy into it.

Fixes: https://github.com/huggingface/transformers/issues/9865

@patil-suraj, @sgugger 
> when you port this to the new run_seq2seq, it would be great to try to find a way to make this not use any special code for a given model

I'm working on it in  #9844, it's not finished though. We might need to add `get_input_embeddings` and `get_pos_embeddings` methods to every s2s model, to avoid special cases.<|||||>If we need to add some methods to deal with the special cases, I would prefer it (otherwise the script might fail with new seq2seq models).