[QoL] Allow dtype str for torch_dtype arg of from_pretrained

[aliencaocao]

What does this PR do?

This is to align with the behavior of pipeline where users can pass strings like torch_dtype="float32" and it gets recognized as torch.float32. It now also happens for any PretrainedModel.from_pretrained.

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [x] Did you read the contributor guideline, Pull Request section?
• [ ] Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• [x] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [x] Did you write any new necessary tests?

Who can review?

@amyeroberts

[aliencaocao]

@amyeroberts added them

[HuggingFaceDocBuilderDev]

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