Add SegRNN

[WenjieDu]

1. Model description

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

@article{lin2023segrnn,
title={SegRNN: Segment Recurrent Neural Network for Long-Term Time Series Forecasting},
author={Shengsheng Lin and Weiwei Lin and Wentai Wu and Feiyu Zhao and Ruichao Mo and Haotong Zhang},
year={2023},
eprint={2308.11200},
archivePrefix={arXiv},
primaryClass={cs.LG}
}

2. Check open-source status

• The model implementation is publicly available

3. Provide useful information for the implementation

https://github.com/lss-1138/SegRNN