fix apply_chat_template() padding choice

[baoyf4244]

What does this PR do?

fix apply_chat_template() padding choice to bool, str, PaddingStrategy and the docstring of pad()

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[Rocketknight1]

The type hint correction is good, but I'm not sure about the docstring! In the code, we pass the padding argument through to PreTrainedTokenizer.encode() unchanged, so I think the documentation of that arg should match the documentation on the underlying tokenizer method

[baoyf4244]

I find in the encode() method, it will pass padding to encode_plus() method, and in this method, it will process padding through _get_padding_truncation_strategies() method. This method do the same with the docstring. And this arg will be finally used in pad() method, which has the same docstring with this arg.

[Rocketknight1]

Good point! Merging, and thanks for the PR!