[WIP] Standardize inputs and outputs for existing image-text-to-text models

[yonigozlan]

What does this PR do?

Pull request aimed at laying the groundwork for adding the image-text-to-text pipeline, while waiting for #31911 to be resolved, by standardizing inputs and outputs for existing image-text-to-text models in Transformers. Follows #29572.

Models whose processors need to be checked/modified:

• GIT
• BLIP
• BLIP-2
• IDEFICS
• InstructBLIP
• LLaVa
• Fuyu
• Pix2Struct/DePlot/MatCha
• UDOP
• Donut
• KOSMOS-2
• Idefics2
• LLaVA-NeXT
• PaliGemma
• VipLlava

If I forgot or misidentified any models, please feel free to add or remove them!

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@zucchini-nlp @amyeroberts @NielsRogge

[HuggingFaceDocBuilderDev]

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