Fix TrainingArguments regression with torch <2.0.0 for dataloader_prefetch_factor

[ringohoffman]

What does this PR do?

dataloader_prefetch_factor was added to TrainingArguments in #28498 with the default value None, but versions of torch<2.0.0 do not accept None and will raise an error if num_workers == 0 and prefetch_factor != 2

Fixes https://discuss.huggingface.co/t/prefetch-factor-issue/74367/4

Also remove a superfluous check that duplicates the <2.0.0 check and the >=2.0.0 check.

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Who can review?

@muellerz @amyeroberts

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[amyeroberts]

Thanks for adding!

Just some small comments

[ringohoffman]

🤔 these failures seem unrelated?

[amyeroberts]

Thanks for iterating!

Normally, I would say we should avoid conditional defaults, but as this keeps the behaviour backwards compatible with torch >= 2 I think the solution is acceptable