Fix MaskFormer/Mask2Former fast image processors (#41393)
* Merge conflict
* add fast processor
* add fast processor
* make style
* add new convert rgb
* use nested group by shape in mllama fast, add support for multiple inputs in group by shape
* fix maskformer mask2 former fast im proc and add tests
* refactor after review
* add _iterate_items utility
* Fix failing tests
* fix copies and improve docs
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Co-authored-by: Vincent <phamvinh257@gmail.com>
