Add new PET Model

# 🌟 New model addition

## Model description

A new article just landed on ArXiv: https://arxiv.org/pdf/2009.07118.pdf
An implementation will eventually be available at https://github.com/timoschick/pet

Authors are @timoschick and Hinrich Schutze.

I didn't see any pre-trained models linked on the GitHub README, but the model is pretty small and easy to train.

Update: the code is available open source along, and it can presumably use pretrained BERT models(I do not know how this works, bu the GitHub page states that the roberta-large pretrained model can be used). The model also works unsupservised.

## Open source status

* [x] the model implementation is available: (give details)
* [x] the model weights are available: (give details)
* [x] who are the authors: (mention them, if possible by @gh-username)