Add overloads for from_pretrained and from_dict on PretrainedConfig
Author: ringohoffman
Pull request: yes

# What does this PR do?

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Related #23980

this is part of a set of changes to improve the type checking in `PreTrainedModel.from_pretrained`

1. Pull `return_unused_kwargs` out of `kwargs` to create `overload`s for `PretrainedConfig.from_pretrained` & `PretrainedConfig.from_dict` for improved return typing
2. add type hints to class variables in `FlaxPreTrainedModel`, `TFPreTrainedModel`, and `PreTrainedModel` so that we can consume the above improvements

before:
<img width="656" alt="Screenshot 2023-06-03 at 10 48 18 PM" src="https://github.com/huggingface/transformers/assets/27844407/d435f97a-bd13-40e4-90f7-43287927aedb">

after:
<img width="660" alt="Screenshot 2023-06-03 at 10 47 31 PM" src="https://github.com/huggingface/transformers/assets/27844407/c0a9e781-6d42-4a44-a3be-ce05c1589b0c">


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## Who can review?

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Documentation: @sgugger, @stevhliu and @MKhalusova

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_23983). All of your documentation changes will be reflected on that endpoint.

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