[Core] Fix get_gpu_ids to return a list of str when CUDA_VISIBLE_DEVICES is set

[jjyao]

Why are these changes needed?

#40286 accidentally changed ray.get_gpu_ids() to always return a list of int while it should return a list of str when CUDA_VISIBLE_DEVICES is set before starting ray.

This PR reverts back to the original behavior.

Related issue number

Closes #43314

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