Support union types `X | Y` syntax for `HfArgumentParser` for Python 3.10+
Author: XuehaiPan
Pull request: yes

# What does this PR do?

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Support union types `X | Y` syntax for `HfArgumentParser` for Python 3.10+. Allow users using Python 3.10+ to opt in new typing futures, such as [union types `X | Y` (PEP 604)](https://peps.python.org/pep-0604). Note that `typing.get_type_hints` does not work for union types on Python 3.7-3.9.

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Fixes #20249


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Testing union types `X | Y` for Python 3.7-3.9 needs to add `from __future__ import annotations` at the top of the test script. I'm not sure should we need to create a separate test script or add new test cases directly in `tests/utils/test_hf_argparser.py`.

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