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

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

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

This is reproducible 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.

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

Signed-off-by: Linkun Chen <github@lkchen.net>

Author: lk-chen