Add ModernTCN

[WenjieDu]

1. Model description

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

@inproceedings{luo2024moderntcn,
title={Modern{TCN}: A Modern Pure Convolution Structure for General Time Series Analysis},
author={Luo Donghao, Wue Xue},
booktitle={The Twelfth International Conference on Learning Representations},
year={2024},
url={https://openreview.net/forum?id=vpJMJerXHU}
}

2. Check open-source status

• The model implementation is publicly available

3. Provide useful information for the implementation

https://github.com/luodhhh/ModernTCN