Add HELIX (ICML 2026 spotlight)

[WenjieDu]

1. Model description

HELIX, accepted as a spotlight paper in ICML 2026, should be integrated into PyPOTS.

@inproceedings{zhang2026helix,
  title={HELIX: Hybrid Encoding with Learnable Identity and Cross-dimensional Synthesis for Time Series Imputation},
  author={Zhang, Fengming and Du, Wenjie and Zhang, Huan and Yu, Ke and Qu, Shen},
  booktitle={International Conference on Machine Learning (ICML)},
  year={2026}
}

2. Check open-source status

• The model implementation is publicly available

3. Provide useful information for the implementation

https://github.com/milaogou/HELIX

4. Your contribution

The 1st author Fengming will make a PR