Change DataCollatorForSeq2Seq to pad labels to a multiple of pad_to_multiple_of

[affjljoo3581]

What does this PR do?

While the input sequences are padded by tokenizer.pad which makes the length of the sequences to be a multiple of pad_to_multiple_of, the label sequences are padded to the maximum length among them without considering pad_to_multiple_of. This PR changes DataCollatorForSeq2Seq class to make sure that the labels have their lengths to be a multiple of the given value.

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Who can review?

@sgugger

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[sgugger]

Great, thanks a lot for adding this!