Enforce pin memory disabling when using cpu only

[qgallouedec]

What does this PR do?

Fixes #26556

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Who can review?

@muellerzr

I would skip writing new test, it's really hard to test, plus it's a minor fix

[muellerzr]

Thanks for the fix!

@ArthurZucker tl;dr summary for you: if use_cpu is used the dataloaders will still use CUDA if available/reserve it in some cases for certain things. pin_memory is a GPU-only capability anyways, so it doesn't matter if the user wants to use their CPU only

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[ArthurZucker]

Thanks both, makes sense