Add return type annotation to PreTrainedModel.from_pretrained
Author: mauvilsa
Pull request: yes

# What does this PR do?

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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.

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Fixes #31850


## Before submitting
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@amyeroberts please review.

Comments: