Add POINTER model

# 🌟 New model addition

## Model description

[POINTER](https://github.com/dreasysnail/POINTER) is a progressive and non-autoregressive text generation pre-training approach, published on EMNLP 2020 by Microsoft Research. POINTER generates fluent text in a progressive and parallel manner. With empirical logarithmic time, POINTER outperforms existing non-autoregressive text generation approaches in hard-constrained text generation.

The model uses basically BERT-large architecture. However, an additional token is added to the vocab. The inference is performed by passing the input iteratively to the model. Since there is no existing model architecture in Huggingface that is compatible, I am not sure how to incorporate this into the model card.

## Open source status

* [x] the model implementation is available: (https://github.com/dreasysnail/POINTER)
* [x] the model weights are available: [here](https://yizzhang.blob.core.windows.net/insertiont/ckpt.tar.gz?st=2020-08-18T20%3A49%3A02Z&se=2024-01-16T20%3A49%3A00Z&sp=rl&sv=2018-03-28&sr=b&sig=PKrSJt38cmY0P%2FBcZuyK%2Btm3bXyYzzfazaqTu1%2F%2FDtc%3D)
* [x] who are the authors: @dreasysnail