[FEA] Rewrite HDBSCAN’s mutual_reachability_knn_l2 with RAFT primitives

[cjnolet]

HDBSCAN contains some code which was adapted directly from the brute force nn code in FAISS to compute a knn directly in reachability space using an array of core distances.

One of the major goals we have had for some time is to remove the FAISS dependency from RAPIDS (specifically cuml and raft) as much as possible to simplify our list of dependencies.

The goal here is to rewrite the logic in mutial_reachabilty_knn_l2 on RAFT primitives so that it no longer requires FAISS. Given how optimized the k-selection code is in FAISS, we may end up ultimately bringing that over to RAFT directly (by copying it out of FAISS but giving proper attribution), but the remaining bits should all be fairly straightforward to port to RAFT directly.

Ideally this would be fixed by rapidsai/raft#798.

[github-actions]

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