[WIP] add DeFormer (ACL 2020) example
Hi there, I'm one of the authors of the [DeFormer paper](https://www.aclweb.org/anthology/2020.acl-main.411/), and I'd like to adapt the [DeFormer codebase](https://github.com/StonyBrookNLP/deformer) to this awesome transformers library. To get the adaptation done, I put a few high-level todos in the README (also here). I plan to get a working example by/before August but don't have a precise timeline yet. Let me hear what you think. Thanks.

- [ ] use HF preprocessing (use HF nlp library)
- [ ] convert original TF DeFormer to HF version
- [ ] convert pre-trained checkpoints 
- [ ] compare and test accuracy for SQuAD, RACE, and BoolQ
- [ ] prepare demo and upload to model cards

Hi Qingqing, good to see you here! Yeah, I checked your official code repo and found it isn't based on 🤗's Transformers. It'll be nice if you can reimplement it and add Deformer in our library! 

~~@thomwolf  Re. `🤗nlp`, should we use `🤗nlp` in `transformers` right away? Since it would make `🤗nlp` a dependency of `transformers/examples`.~~

Never mind, it already is.<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
