Add Mask2Former
### Model description

Mask2Former is a single architecture for panoptic, instance and semantic segmentation.

**Mask2Former Paper Abstract**:
Image segmentation is about grouping pixels with different semantics, e.g., category or instance membership, where each choice of semantics defines a task. While only the semantics of each task differ, current research focuses on designing specialized architectures for each task. We present Masked-attention Mask Transformer (Mask2Former), a new architecture capable of addressing any image segmentation task (panoptic, instance or semantic). Its key components include masked attention, which extracts localized features by constraining cross-attention within predicted mask regions. In addition to reducing the research effort by at least three times, it outperforms the best specialized architectures by a significant margin on four popular datasets. Most notably, Mask2Former sets a new state-of-the-art for panoptic segmentation (57.8 PQ on COCO), instance segmentation (50.1 AP on COCO) and semantic segmentation (57.7 mIoU on ADE20K).


### Open source status

- [X] The model implementation is available
- [X] The model weights are available

### Provide useful links for the implementation

Paper: https://arxiv.org/abs/2112.01527

Github repo (and weights): https://github.com/facebookresearch/Mask2Former
@NielsRogge I'd like to work on adding this model if no one is working on it yet?<|||||>cc'ing @alaradirik, yes we're planning to add this model. If you're interested in it, feel free to get started with a draft PR. Note that we already have [MaskFormer](https://github.com/huggingface/transformers/tree/main/src/transformers/models/maskformer) implemented, and I've heard Mask2Former only adds minor modifications.

Could you give me your email address, such that we can add you on Slack for easier communication?<|||||>Thanks @NielsRogge that would be great! You can use this email (shivalikasingh95@gmail.com) to add me on Slack!

I'll get started on a draft PR. But, I may need some guidance as this is my first time contributing to transformers.
I'll get started by understanding the [MaskFormer](https://github.com/huggingface/transformers/tree/main/src/transformers/models/maskformer) implementation.<|||||>Hi @NielsRogge just a gentle reminder to add me on slack :)<|||||>Hi, I've pinged someone to add you.<|||||>Hello.
Any updates about the Mask2Former integration? Thanks<|||||>Hi @ArthurOuaknine I'm working on it. Currently there is an open PR on my transformers fork. Will try to close this in next couple of days. <|||||>Thanks for your work, it will definitely help :) 