Add Reformer

[WenjieDu]

1. Model description

Reformer should be included in PyPOTS, and could start from the imputation task.

@inproceedings{kitaev2020reformer,
title={Reformer: The Efficient Transformer},
author={Nikita Kitaev and Lukasz Kaiser and Anselm Levskaya},
booktitle={International Conference on Learning Representations},
year={2020},
url={https://openreview.net/forum?id=rkgNKkHtvB}
}

2. Check open-source status

• The model implementation is publicly available

3. Provide useful information for the implementation

https://github.com/google/trax/tree/master/trax/models/reformer and https://github.com/lucidrains/reformer-pytorch