

TITLE
[WIP] add MobileViT model

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# What does this PR do?

Add the MobileViT model to Transformers. This is a computer vision model that combines CNNs with transformers: https://machinelearning.apple.com/research/vision-transformer

The model comes in three sizes: small, extra small, and xx-small. There are three heads: image classification, object detection, and semantic segmentation.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case. (Internal discussion on Slack.)
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?

## TODO

- [ ] write tests
- [ ] verify that finetuning a model works correctly
- [ ] deal with the original license
- [ ] ONNX config
- [ ] add object detection (SSD) model
- [ ] add semantic segmentation (DeepLab) model


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

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