[VLM][Bugfix] Make sure that multi_modal_kwargs is broadcasted properly

[xwjiang2010]

Fix #5863

The current broadcasting logic only supports first level tensors. For multi_modal_kwargs, tensors can be nested under.
The PR extends tensor_dict flatten and unflatten functionality to be more generic (handles the above case).

Also added a unit test.

cc @ywang96 @DarkLight1337

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[ywang96]

cc @youkaichao for awareness

[ywang96]

@xwjiang2010 Should we also remove the code to move pixel_values to device code here once this PR is merged? Maybe we should make this change in this PR as well?

vllm/vllm/model_executor/models/llava.py

Line 194 in b9e8425

image_features = vision_tower(pixel_values.to(vision_tower.device),

[DarkLight1337]

Let's merge this first. I'll work on adding TP tests for vision models - we can then test whether removing that code is safe.

[youkaichao]

We should raise exception when the original keys contain % .