Improve `.from_pretrained` type annotations
Author: qubvel
Pull request: yes

# What does this PR do?

Currently, the return type annotation of the `from_pretrained` method is a generic `PreTrainedModel`. This results in a suboptimal experience when accessing model-specific methods, their parameters, and corresponding docstrings. This PR improves it

---
**Here is an example from VSCode IDE on `main`:**

<img width="400" alt="Screenshot 2024-11-27 at 14 32 45" src="https://github.com/user-attachments/assets/8f0091af-1f0d-4834-8829-f456a598ae1a">
<img width="443" alt="Screenshot 2024-11-27 at 14 32 37" src="https://github.com/user-attachments/assets/0ce0167e-e167-4a83-8481-b54aafdcb382">

---

**Here is a version with corrected type annotation:**

<img width="416" alt="Screenshot 2024-11-27 at 14 28 53" src="https://github.com/user-attachments/assets/fd136c1d-bb15-4f14-8dcd-d22e5cec57db">
<img width="618" alt="Screenshot 2024-11-27 at 14 28 40" src="https://github.com/user-attachments/assets/33e74a43-a4b0-4ceb-a72d-6374c48eba57">



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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_34973). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

This is nice. But could you explain a bit how it works for someone don't know this thing at all 🙏 ?

Hey @ydshieh2! I would recommend looking into 
 - https://docs.python.org/3.11/library/typing.html#typing.TypeVar
 - https://guicommits.com/python-generic-type-function-class/
 
it should be helpful!

Yes, Type and TypeVar both exist in python3.9

- https://github.com/python/cpython/blob/fb0b642bf1aa3ec276304b7170deedd5040c1698/Lib/typing.py#L1750
- https://github.com/python/cpython/blob/fb0b642bf1aa3ec276304b7170deedd5040c1698/Lib/typing.py#L581

Cool! Feel free to merge then!