Add LLaVA model to transformers

[mattmazzola]

What does this PR do?

Add LLaVA model to transformers
There are 4 pretrained models

• liuhaotian/llava-v1.5-7b
• liuhaotian/llava-v1.5-13b
• liuhaotian/llava-v1.5-7b-lora
• liuhaotian/llava-v1.5-13b-lora

Fixes # (issue)

#22848

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

• text models: @ArthurZucker and @younesbelkada
• vision models: @amyeroberts

[younesbelkada]

Hi @mattmazzola
Thank you very much for your great work! I think that there is already a contributor working on adding Llava in transformers: #25789 - perhaps you can follow up with them and check the current status on that PR and see how you can collaborate? 🙏