Request addition of 'GPT2ForwardBackward' models

# 🌟 Request addition of 'GPT2ForwardBackward' models

## Model description

Code for running forward and backward versions of GPT-2 XL. These were trained for the paper:

**Reflective Decoding: Beyond Unidirectional Generation with Off-the-Shelf Language Models**; Peter West, Ximing Lu, Ari Holtzman, Chandra Bhagavatula, Jena Hwang, and Yejin Choi; ACL (2021)

https://arxiv.org/abs/2010.08566

## Open source status

* [ X] the model implementation is available: (https://github.com/peterwestuw/GPT2ForwardBackward)
* [ X] the model weights are available: (same link as above)
* [ X] who are the authors: (See arvix credits above)