Large change to enable MaskFormerForInstanceSegmentation
Author: Narsil
Pull request: yes

# What does this PR do?

Changing the goal of this PR.

I'll split this PR into more manageable chunks instead:

- Adding the AutoDicts : https://github.com/huggingface/transformers/pull/15934
- Making modifications for FeatureExtractor https://github.com/huggingface/transformers/pull/15916
- Making modifications (breaking change) to MaskFormerForInstance (for the return outputs to be None instead of empty list) https://github.com/huggingface/transformers/pull/15936
- Finally a PR for the pipeline that depends on the previous 3 (which are not ordered themselves). https://github.com/huggingface/transformers/pull/15937
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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_15923). All of your documentation changes will be reflected on that endpoint.

closing in favor of https://github.com/huggingface/transformers/pull/15937