Add TimeMixer

[WenjieDu]

1. Model description

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

@inproceedings{wang2024timemixer,
title={TimeMixer: Decomposable Multiscale Mixing for Time Series Forecasting},
author={Shiyu Wang and Haixu Wu and Xiaoming Shi and Tengge Hu and Huakun Luo and Lintao Ma and James Y. Zhang and JUN ZHOU},
booktitle={The Twelfth International Conference on Learning Representations},
year={2024},
url={https://openreview.net/forum?id=7oLshfEIC2}
}

2. Check open-source status

• [X] The model implementation is publicly available

3. Provide useful information for the implementation

https://github.com/kwuking/TimeMixer