Add return type annotation to PreTrainedModel.from_pretrained

[mauvilsa]

What does this PR do?

As the title says, this adds the return type annotation to PreTrainedModel.from_pretrained. This is necessary so that pytorch-lightning's LightningCLI is able to make automatically configurable transformer models.

Fixes #31850

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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@amyeroberts please review.

[amyeroberts]

Thanks for adding!