[FEA] Use RBC / IVF to perform kernel density queries.

[cjnolet]

This should help us lower the storage requirement of Kernel Density estimation from n^2.

Ref: rapidsai/cuml#4545

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This issue has been labeled inactive-90d due to no recent activity in the past 90 days. Please close this issue if no further response or action is needed. Otherwise, please respond with a comment indicating any updates or changes to the original issue and/or confirm this issue still needs to be addressed.