Compute inverse propensity scores based on a label distribution. Propensity
scores for extreme multi-label learning are proposed in Jain, H., Prabhu, Y.,
& Varma, M. (2016). Extreme Multi-label Loss Functions for Recommendation,
Tagging, Ranking and Other Missing Label Applications. Proceedings of the
22nd ACM SIGKDD International Conference on Knowledge Discovery and Data
Mining, 13-17-Aug, 935–944. tools:::Rd_expr_doi("10.1145/2939672.2939756").