Faulty comms in Dask kNN #7573
Description
Since ucx-py was removed from the environment, Dask NearestNeighbors, KNeighborsClassifier and KNeighborsRegressor started to produce gibberish outputs resulting in low accuracy in tests. This was caused by issues in the RAFT communicator and is probably related to the removal of the UCX-py backend in favor of UCXX. Even though the UCX build in the environment is CUDA-aware, only the communications with pointers on host memory were transmitted successfully.
Since these estimators only exchange device memory, bypassing UCX and using the NCCL library directly (still through the RAFT communicator though) would solve the issue at least temporarily.