Clean up dist import

[muellerzr]

What does this PR do?

Cleans up the torch.distributed.X imports in training_args to use the already imported dist module, which helps clean up our logic and cases quite a bit.

Fixes # (issue)

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@amyeroberts (cc @sgugger )

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[amyeroberts]

Thanks for tidying up!