[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.

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• Did you write any new necessary tests?

Who can review?

@amyeroberts

[amyeroberts]

Thanks for adding! I think this looks OK, but let's get a second opinion from @ArthurZucker as this touches a core part of the modeling logic.

We should also add tests - probably under test_modeling_utils.py

[aliencaocao]

@amyeroberts added them

[amyeroberts]

Thanks for adding the tests!

[ArthurZucker]

Nice nice

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