[Model] Expose size to Idefics3 as mm_processor_kwargs

[Isotr0py]

• Expose size to initialize Idefics3Processor/Idefics3ImageProcessor, which can be used to reduce resized image size and save memory: https://huggingface.co/HuggingFaceM4/Idefics3-8B-Llama3#model-optimizations

cc @jeejeelee @B-201

[github-actions]

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[jeejeelee]

This is unrelated to this PR, but I feel the test script names in this directory are a bit unreasonable.

[Isotr0py]

Perhaps we can consolidate the mm_processor_kwargs tests as well?

[DarkLight1337]

Let's do that in a separate PR.

[jeejeelee]

Perhaps we can consolidate the mm_processor_kwargs tests as well?

Good idea. Perhaps https://github.com/vllm-project/vllm/tree/main/tests/multimodal is a better choice