Create a NCCL sub-communicator using ncclCommSplit #2495
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NCCL expects to create a sub-communicator using
ncclCommSplit(like we create a MPI sub-communicator usingMPI_Comm_split) but raft currently creates a sub-communicator from scratch usingncclCommInitRankon a subset of GPUs.This first prevents sharing resources among the global communicator and sub-communicators. We also experienced erroneous sub-communicator behaviors when we have a non-power-of-two number of GPUs.
This PR updates raft to create NCCL sub-communicators using
ncclCommSplit.