

TITLE
Tokenizers should be framework agnostic

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2

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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.