[Core][VLM] Stack multimodal tensors to represent multiple images within each prompt

[petersalas]

As part of supporting multiple {image, audio} inputs per prompt, this generalizes multi-modal input batching to stack inputs so as to create dimensions for both multiple prompts and multiple images within each prompt. The various model implementations either squeeze the new dimension (for those that don't yet support multiple images) or flatten it with the batch the dimension (for those that do).

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[github-actions]

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[DarkLight1337]

Thanks for helping out with this! Please take a look at the above comments.

[DarkLight1337]

LGTM, thanks for implementing!