Use _PATH in annotations and convert to str if Path

[dyollb]

What does this PR do?

This minor change adds type annotation for pathlib.Path to some code locations, e.g. Trainer(default_root_dir: _PATH.
The type _PATH which is Union[str, Path] is used as it is already declared/used elsewhere.

To not break downstream code that expects str, the _PATH args/options are converted to str using os.fspath.

Fixes #15497

Does your PR introduce any breaking changes? If yes, please list them.

None

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Writing a test for these small changes doesn't seem useful.

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