revert select_device

[Jintao-Huang]

Pre-initializing CUDA causes environment variables to become ineffective.

[gemini-code-assist]

Code Review

This pull request replaces the custom select_device utility with direct assignments to os.environ['CUDA_VISIBLE_DEVICES'] across numerous test files. The review feedback identifies several key improvement opportunities: first, CUDA_VISIBLE_DEVICES should be set before importing torch to avoid premature CUDA initialization; second, environment variables should be set at the module level rather than inside if __name__ == '__main__': blocks so they are properly configured when run via test runners like pytest; and third, using os.environ.setdefault is recommended to allow command-line overrides in local or CI/CD environments.

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