Remove redundant code from TrainingArgs

[muellerzr]

What does this PR do?

Removes some more redundant code that Accelerate can handle directly. Namely:

• World size
• Process index
• main_process_first

Fixes # (issue)

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you make sure to update the documentation with your changes? Here are the
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• Did you write any new necessary tests?

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@sgugger @pacman100

[HuggingFaceDocBuilderDev]

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

[sgugger]

Thanks!