[Feature] Support mistralai/Pixtral

[merrymercy]

Motivation

mistralai/Pixtral is one of the best vision language models. We would like to support it.

Models:
https://huggingface.co/mistralai/Pixtral-12B-2409
https://huggingface.co/mistralai/Pixtral-Large-Instruct-2411

Related resources

You can learn from the existing implementations and usage examples of other vision language models.
https://sgl-project.github.io/references/supported_models.html#how-to-support-a-new-model
https://github.com/sgl-project/sglang/blob/main/python/sglang/srt/models/llava.py
https://github.com/sgl-project/sglang/blob/main/test/srt/test_vision_openai_server.py
https://sgl-project.github.io/references/sampling_params.html#multi-modal

[yixin-huang1]

drafted a PR.

[DanielC12321]

I will give this a try if no one else is working on it.

[raayandhar]

Hi, will be giving this a try.

[b8zhong]

Completed in #5084