Change DataCollatorForSeq2Seq to pad labels to a multiple of `pad_to_multiple_of`
Author: affjljoo3581
Pull request: yes

# What does this PR do?

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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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