[Misc] Add example for LLaVA-NeXT

[ywang96]

There has been some user confusion (e.g, #1751 (comment), #4194 (comment), #4199 (comment)) on how to serve and use LLaVA-NexT on vLLM even though the support for it is currently experimental.

This PR adds an example with a note on its current limitation.

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[someshsingh22]

Hi, how do I do this for a batch of images / prompts ?

[ywang96]

Hi, how do I do this for a batch of images / prompts ?

Instead of passing a dictionary, you would pass a list of dictionaries like the following

outputs = llm.generate(
    [{
        "prompt": prompt,
        "multi_modal_data": ImagePixelData(image),
    } for _ in range(10)],
    sampling_params=sampling_params)