Add `segmentation_maps` support to MobileNetV2ImageProcessor
Author: simonreise
Pull request: yes

MobileNetV2 can do semantic segmentation, but its ImageProcessor can not process `segmentation_maps`.

This PR adds `segmentation_maps` support to MobileNetV2ImageProcessor. It also adds `reduce_labels` support and restructures old `preprocess` function to separate `_preprocess`, `_preprocess_image` and `_preprocess_mask` functions just like it is done e.g. in SegFormer image processor.

This PR also adds `reduce_labels` support to MobileViTImageProcessor to keep it consistent with other image processors.

I also added the required tests.

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I also tried to add `segmentation_maps` support to the new MobileNetV2 `FastImageProcessor` in the recent commits. 

Tried to run imageprocessor tests and they passed, but more detailed code review is definetely needed in this case.

cc @qubvel @nielsrogge

Changed Fast Image Processor logic to match the newly merged BEIT Fast Image Processor. Could someone review, please?

Also added `reduce_labels` support to the newly merged MobileVit Fast image processor

Now the PR is totally ready for the review
@qubvel @NielsRogge maybe @yonigozlan 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_37312). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

**[For maintainers]** Suggested jobs to run (before merge)

run-slow: mobilenet_v2, mobilevit