Uniformize kwargs for Idefics/2 processors
Author: yonigozlan
Pull request: yes

# What does this PR do?
Adds uniformized processors kwargs following https://github.com/huggingface/transformers/issues/31911 for the following models:
- [x] Idefics
- [x] Idefics2
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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_32568). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

This should now be ready for review! Cc @molbap @amyeroberts .
It also brings some changes to test_processing_common that could benefit other models.
I think with @molbap PR https://github.com/huggingface/transformers/pull/31368 and the following pending PRs https://github.com/huggingface/transformers/pull/32544 https://github.com/huggingface/transformers/pull/33668 https://github.com/huggingface/transformers/pull/32181 merged, that would be all the image-text-to-text processors uniformized!

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Hey @ArthurZucker !
This should be ready for a final review.
A little overview of the changes:
- Idefics processor needed quite a lot of added logic to work with the new standardized processor signature, as it used to take prompts inputs only and not separate between images and text, but full BC should be supported still.
- Idefics2 processor uniformization is quite straightforward, and is now very close to Idefics3 processor.
- Idefics processor tests needed a bit of overriding as it is very different from other vlms processor in Transformers, and notably doesn't take in `do_rescale` or `scale_factor` as args for its call function, which we use in common tests.
- Idefics2 processor tests needed only overriding the mock inputs, as the text prompts need to include the <image> token for each corresponding input images, and the processor needs nested images when working with batched inputs.
Thanks you!