Add cross-correlation as imputation evaluation metric

[rvandewater]

See:
Medical Data Wrangling With Sequential Variational Autoencoders,' in IEEE Journal of Biomedical and Health Informatics, vol. 26, no. 6, pp. 2737-2745, June 2022, doi: 10.1109/JBHI.2021.312383,

GitHub repo:
https://github.com/dbarrejon/Shi-VAE/blob/e03a1a727df0489b67d23cd53664fadadc5b3032/src/lib/loss.py#L18