Add UniTS

[WenjieDu]

1. Model description

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

@article{gao2024units,
title={{UniTS}: Building a Unified Time Series Model},
author={Gao, Shanghua and Koker, Teddy and Queen, Owen and Hartvigsen, Thomas and Tsiligkaridis, Theodoros and Zitnik, Marinka},
journal={arXiv},
url={https://arxiv.org/pdf/2403.00131.pdf},
year={2024}
}

2. Check open-source status

• The model implementation is publicly available

3. Provide useful information for the implementation

https://github.com/mims-harvard/UniTS