[core][refactor] Move accelerator-specific environment variables to ray_constants.py to avoid redefining them

[kevin85421]

Why are these changes needed?

Env vars like CUDA_VISIBLE_DEVICES_ENV_VAR are defined in both ray_constants.py and nvidia_gpu.py. This PR moves related env vars to ray_constants.py to avoid redefining them.

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[kevin85421]

This is the same as ASCEND_RT_VISIBLE_DEVICES_ENV_VAR.

[edoakes]

Hm I think it's preferable to keep accelerator-specific logic isolated in the relevant accelerators/ file to avoid polluting the top-level Ray core namespace.

If we want to avoid redefining commonly-used ones, we can import them into ray_constants as well.

@jjyao WDYT?

[kevin85421]

Hm I think it's preferable to keep accelerator-specific logic isolated in the relevant accelerators/ file to avoid polluting the top-level Ray core namespace.

Note that some environment variables are used not only by xxxxxxpu.py but also by other files, which import them from ray_constants.py. For example, you can search CUDA_VISIBLE_DEVICES_ENV_VAR.

If we want to avoid redefining commonly-used ones, we can import them into ray_constants as well.

In my opinion, it's better not to import any Ray-related dependencies into ray_constants.py to avoid potential circular dependency issues in the future. We should aim to keep it as a top-level Python module in Ray.

[kevin85421]

Another way is to create accelerator_constants.py and it doesn't import any other Ray Python modules. All files want to use the env vars need to import the file.