add MobileNetV1 model
Author: hollance
Pull request: yes
Labels: name: New model

# What does this PR do?

Adds the MobileNet V1 model to the library.

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## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [x] Did you write any new necessary tests?


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Models:

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- pipelines: @LysandreJik

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Note: The checkpoints still point to my own account, but should be changed to `google` once the changes are approved.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17799). 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_17799). All of your documentation changes will be reflected on that endpoint.

I've rebased it so that it can merged. Perhaps @sgugger or @NielsRogge could merge it?