Add ImputeFormer

[WenjieDu]

1. Model description

ImputeFormer should be included in PyPOTS as an imputation model.

@article{nie2023imputeformer,
  title={ImputeFormer: Low Rankness-Induced Transformers for Generalizable Spatiotemporal Imputation},
  author={Nie, Tong and Qin, Guoyang and Ma, Wei and Mei, Yuewen and Sun, Jian},
  journal={arXiv preprint arXiv:2312.01728},
  year={2023}
}

2. Check open-source status

• The model implementation is publicly available

3. Provide useful information for the implementation

https://github.com/tongnie/ImputeFormer

[WenjieDu]

We are currently using SAITS loss to train ImputeFormer in PyPOTS. Tong @tongnie will make a PR to apply the original loss function of ImputeFormer soon.