Add support for custom inputs and batched inputs in ProcessorTesterMixin
Author: yonigozlan
Pull request: yes

# What does this PR do?
Add support for custom inputs and batched inputs in `ProcessorTesterMixin` to avoid rewriting tests.
Can be useful for models who need special text input formatting (e.g. idefics 2 needs `<image>` in prompt for each corresponding image inputs).
Can also be useful to tests different batching behaviors.

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Fixes # (issue)


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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_33711). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

@amyeroberts I requested a re-review because I made quite a few changes to other processors/test_processors as a consequence of introducing this new behavior :)