add MobileNetV2 model
Author: hollance
Pull request: yes
Labels: name: New model

# What does this PR do?

Adds MobileNetV2 to the Transformers library.

This includes an image classification head and a basic DeepLabV3+ semantic segmentation head.

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Fixes # (issue)


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- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

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- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

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HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17845). All of your documentation changes will be reflected on that endpoint.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17845). All of your documentation changes will be reflected on that endpoint.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17845). All of your documentation changes will be reflected on that endpoint.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17845). All of your documentation changes will be reflected on that endpoint.

Good to merge @hollance ?

@sgugger It seems to fail some tests but they don't look they're from my changes? If you're OK with that test failing then I think this is ready to get merged.

Failure is a flaky test, unrelated to this PR.