What does this PR do?

Fixes #3516.

Fixes the logic in accelerator._prepare_ipex such that:

• Multiple nn.Module objects can now be prepared in parallel, without silent failures.
• An error is raised if a user attempts to prepare a combination that is non-trivial to infer (e.g., two modules and two optimizers at the same time).
• A cautionary note and a suggested workaround for this limitation (due to intel-extension-for-pytorch) is added to the documentation.

Before submitting

• 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?

Who can review?

@yao-matrix @BenjaminBossan @SunMarc @zach-huggingface

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Co-authored-by: Marc Sun <57196510+SunMarc@users.noreply.github.com>

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The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.