refactor: use __all__ in accelerators/__init__.py

[littlebullGit]

What does this PR do?

This PR refactors the [accelerators/init.py]file to use Python's __all__ for explicit exports instead of using # noqa: F401 comments. This change makes the public API more explicit and follows Python best practices.

Key Changes:

• Added __all__ list at the top of the file to explicitly define exported symbols
• Removed # noqa: F401 comments from imports
• Maintained all existing functionality while improving code quality

Motivation:

• Makes the public API more explicit and self-documenting
• Follows Python best practices for module exports
• Improves code maintainability and IDE support
• Removes linter workarounds

Fixes #11592

Before submitting

• Was this discussed/agreed via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (not needed)
• Did you write any new necessary tests? (not needed)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request? (none)
• Did you update the CHANGELOG? (not needed for this refactor)

Notes:

• This is a non-breaking change that only affects internal imports
• No functionality has been changed
• All existing tests pass with these changes

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📚 Documentation preview 📚: https://pytorch-lightning--20889.org.readthedocs.build/en/20889/