Add Non-stationary Transformer

[WenjieDu]

1. Model description

Non-stationary Transformer should be included in PyPOTS, and could start from the imputation task.

@inproceedings{liu2022nonstationary,
author = {Liu, Yong and Wu, Haixu and Wang, Jianmin and Long, Mingsheng},
booktitle = {Advances in Neural Information Processing Systems},
editor = {S. Koyejo and S. Mohamed and A. Agarwal and D. Belgrave and K. Cho and A. Oh},
pages = {9881--9893},
publisher = {Curran Associates, Inc.},
title = {Non-stationary Transformers: Exploring the Stationarity in Time Series Forecasting},
url = {https://proceedings.neurips.cc/paper_files/paper/2022/file/4054556fcaa934b0bf76da52cf4f92cb-Paper-Conference.pdf},
volume = {35},
year = {2022}
}

2. Check open-source status

• The model implementation is publicly available

3. Provide useful information for the implementation

https://github.com/thuml/Nonstationary_Transformers