Mamba2 conversion script for original models
Author: vasqu
Pull request: yes

# What does this PR do?
Extends the Mamba2 conversion script to be compatible with the paper models and codestral. I need some help handling the tokenizer or more specifically how I can overwrite the padding side of a pretrained tokenizer and then save it with the new side.

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Fixes #32496


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@ArthurZucker @molbap 

Comments:
Tests seem unrelated to me. I'll rebase after resolving everything in the review. 

Yes there are issues with tests on main right now, on it! unrelated to your changes. To choose the model to convert I think it should be user-dependent - meaning, it could simply be a `choice` argument in the parser and that's it, right?

Oh yea for sure, I kinda just went ahead and tried to automate everything :facepalm: Lemme change it in the evening when I have proper time!

With all the fixes/patches in previous PRs, this PR should be ready now. 

I think there are still two core things to consider:
- Is the addition of the `Mamba2Tokenizer` fine?
- Is the model dictionary `_MAMBA2_MODELS_DICT` fine with the partial functions or is this overengineered? :eyes: 

Feel free to ping me for another review once ready! 

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@ArthurZucker Should be good for review :eyes: just checked locally to make sure it works.