[Orbit] Fix type hints in new_best_metric.py

[LINYV0719]

…sing return types

Description

The filename argument in JSONPersistedValue was annotated as str, but the implementation explicitly checks if self._filename is not None, implying it should be Optional[str] This PR: Changes filename: str to filename: Optional[str]. Adds missing return type hints (-> Any, -> None) for read and write methods. Verified with python -m orbit.actions.new_best_metric_test.

Type of change

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Note: Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] Documentation update
• [ ] TensorFlow 2 migration
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] A new research paper code implementation
• [ ] Other (Specify)

Tests

I ran the existing unit tests to verify that the type hint changes didn't break any functionality.

Test Configuration:

OS: Windows Python Version: Python 3.10 Command: python -m orbit.actions.new_best_metric_test Result: All tests passed.

Checklist

• [x] I have signed the Contributor License Agreement.
• [x] I have read guidelines for pull request.
• [x] My code follows the coding guidelines.
• [x] I have performed a self code review of my own code.
• [ ] I have commented my code, particularly in hard-to-understand areas.
• [x] I have made corresponding changes to the documentation.
• [x] My changes generate no new warnings.
• [x] I have added tests that prove my fix is effective or that my feature works.