Weighted version of the KLL sketch?

Hello,

There's a consideration at [XGBoost](https://github.com/dmlc/xgboost/issues/5746) about potentially using the KLL sketch to represent feature value histograms.

One potential blocker is the need for a weighted version of the sketch, this would allow us to use data points that are weighted, and adjust their feature contributions accordingly (See Appendix A of [XGBoost paper](https://arxiv.org/abs/1603.02754)).

I remember discussing in the past the possibility of using data weights with KLL, is that still an option?

Inizia esaminando il design dello sketch KLL e la issue #5746 di XGBoost collegata, quindi leggi l’Appendix A del paper di XGBoost collegato per i requisiti di ponderazione. Determina se uno sketch KLL pesato è fattibile e definisci il comportamento previsto prima dell’implementazione; la issue non indica file sorgente né test.

Scritto dal modello di indicizzazione a partire dal testo della issue.