fix variable undefined bug when return_tensors is not specified in llava processing

[chenweize1998]

What does this PR do?

Fixes #34952 (issue)

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[zucchini-nlp]

Hey, thanks a lot for fixing this! I had it somewhere on a bigger PR to clean up all VLMs and ensure it works, but I think we can merge this now

Left one tiny comment but overall LGTM

[chenweize1998]

Just revised the code according to the review.

[zucchini-nlp]

Core maintainer's approve and we'll merge it!

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