[Bugfix] Validate and concatenate image embeddings in MiniCPMVBaseModel

[whyiug]

When performing batch embedding on different images, list needs to be converted.
FIX #11630

[github-actions]

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[DarkLight1337]

Thanks for fixing!

(Note to others: we don't need to preserve the batch dimension since merge_multimodal_embeddings doesn't really care about that anyway)