Use non nested images and batched text Idefics2/3 
Author: yonigozlan
Pull request: yes

# What does this PR do?
Unlike most other image-text-to-text models, Idefics2 and Idefics3 do not support inputs such as:
`images=[image1, image2], text=["text1 <image>", "text2 <image>"]`
But needs images to be nested to follow the number of tokens in each prompt:
`images=[[image1], [image2]], text=["text1 <image>", "text2 <image>"]`

This PR adds support for the former, and also for deducing the nesting of a flat image lists to follow the number of image tokens in each prompt, meaning that this is also supported:
`images=[image1, image2, image3], text=["text1 <image>", "text2 <image><image>"]`

This is done mostly to have a consistent batching behavior in the [image-text-to-text pipeline](https://github.com/huggingface/transformers/pull/34170) between Idefics2/3 and other models.

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@andimarafioti 

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_34222). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Added tests @ArthurZucker :)