Add Koopa

[WenjieDu]

1. Model description

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

@inproceedings{liu2023koopa,
author = {Liu, Yong and Li, Chenyu and Wang, Jianmin and Long, Mingsheng},
booktitle = {Advances in Neural Information Processing Systems},
editor = {A. Oh and T. Neumann and A. Globerson and K. Saenko and M. Hardt and S. Levine},
pages = {12271--12290},
publisher = {Curran Associates, Inc.},
title = {Koopa: Learning Non-stationary Time Series Dynamics with Koopman Predictors},
url = {https://proceedings.neurips.cc/paper_files/paper/2023/file/28b3dc0970fa4624a63278a4268de997-Paper-Conference.pdf},
volume = {36},
year = {2023}
}

2. Check open-source status

• The model implementation is publicly available

3. Provide useful information for the implementation

https://github.com/thuml/Koopa