Add FreTS

[WenjieDu]

1. Model description

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

@inproceedings{yi2023frets,
author = {Yi, Kun and Zhang, Qi and Fan, Wei and Wang, Shoujin and Wang, Pengyang and He, Hui and An, Ning and Lian, Defu and Cao, Longbing and Niu, Zhendong},
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 = {76656--76679},
publisher = {Curran Associates, Inc.},
title = {Frequency-domain MLPs are More Effective Learners in Time Series Forecasting},
url = {https://proceedings.neurips.cc/paper_files/paper/2023/file/f1d16af76939f476b5f040fd1398c0a3-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/aikunyi/FreTS