Add iTransformer

[WenjieDu]

1. Model description

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

@inproceedings{liu2024itransformer,
title={{iTransformer: Inverted Transformers Are Effective for Time Series Forecasting}},
author={Yong Liu and Tengge Hu and Haoran Zhang and Haixu Wu and Shiyu Wang and Lintao Ma and Mingsheng Long},
booktitle={The Twelfth International Conference on Learning Representations},
year={2024},
url={https://openreview.net/forum?id=JePfAI8fah}
}

2. Check open-source status

• The model implementation is publicly available

3. Provide useful information for the implementation

https://github.com/thuml/iTransformer