[WIP] Add PoNet

[lxchtan]

What does this PR do?

Implementation of PoNet model (https://arxiv.org/abs/2110.02442).

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, Pull Request section?
• [ ] Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• [x] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [x] Did you write any new necessary tests?

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.

@ArthurZucker and @younesbelkada.

[sgugger]

cc @ArthurZucker and @younesbelkada

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[ArthurZucker]

Hey! great work🔥

Would you be open to put this model on the hub following this tutorial! This model seems very similar to a Bert model, so it makes more sense! Especially for all the additional ressources that you want to add

[lxchtan]

Thanks for your advice! I've followed the tutorial and put the codes to the hub.

[github-actions[bot]]

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