Tokenizers should be framework agnostic
The `prepare_seq2seq_batch` method should not return PyTorch tensors by default. It does not in the base class, and all our tokenizer methods should be agnostic to the framework.

Updated Marian, Pegasus, mBART, FSMT that had `return_tensors="pt"` and RAG that had `return_tensors="np"`.

The documentation for these methods was inconsistent, added the docstrings via the decorator where it was needed.
Can't ask you to review here @stas00 but would love your review as FSMT is impacted by this.<|||||>LGTM