Fix llava_instruct_dataset

[RdoubleA]

Context

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Changelog

What are the changes made in this PR?

• Process multiturn conversations with one image correctly (image only included in first user message)
• Remove train on input from all multimodal datasets since image id is out of vocab

Test plan

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UX

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[pytorch-bot]

🔗 Helpful Links

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As of commit 0f6edba with merge base 17f0bff ():
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