[vjkumar2756]

Description

This PR addresses #640 by improving the TrainerClient implementation.

Changes made

• Added missing return type annotations for improved type safety.
• Replaced if not backend_config with if backend_config is None for more explicit None checking.
• Improved docstrings for better clarity and consistency.
• Enhanced error messages for unsupported backend configurations.
• Improved overall code readability and maintainability.

Testing

• Verified that existing functionality remains unchanged.
• Ran the relevant test suite to ensure all tests pass.

Fixes #640

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[lntutor]

The proposed get_runtime_packages(...) -> str annotation does not match the backend implementations. LocalProcessBackend.get_runtime_packages() returns local_runtime.trainer.packages, and kubeflow/trainer/backends/localprocess/backend_test.py explicitly asserts it is a list; the container and Kubernetes implementations print their results and return None. Please use the verified cross-backend return type (for example list[str] | None, if package entries are strings) or leave this method unannotated until the backend interface is made consistent. Also add a delegation test for TrainerClient so the public annotation is protected by the actual API behavior.

[andreyvelich]

@vjkumar2756 Sorry for the delay, can you rebase this PR, so we can merge it?
/ok-to-test