[V1][Molmo] Fix get_multimodal_embeddings() in molmo.py

[lk-chen]

Expected: get_multimodal_embeddings() should return list[Tensor] for GPUModelRunner to iterate.

Actual: prior to this PR, molmo's _get_mm_embeds() returns a list thus get_multimodal_embeddings() returns a list of list.

The issue is surfaced when all of following satisfy:

• more than one request
• the tailing part of each request is a bit different, to trigger partial cache hit

This PR also updates vision_language.py to help reproduce above issue.

Tested with:

VLLM_USE_V1=1 \
python examples/offline_inference/vision_language.py \
  --model molmo \
  --num-prompts=2 \
  --use-different-prompt-per-request

cc @DarkLight1337 @ywang96

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[mergify[bot]]

This pull request has merge conflicts that must be resolved before it can be merged. Please rebase the PR, @lk-chen.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork