Use One Slice per Basic Primitive for AllReduce, ReduceScatter, AllGather (#1681) for Single Node on Some GFX9 Systems

Using a single slice rather than the typical two provides about 5% speedup (sometimes more or less) on some GFX9 systems for single node.

src/include/comm.h

@@ -481,6 +481,7 @@ struct ncclComm {
 
   int node;
   int nNodes;
+  int rcclUseOneSlice; // RCCL: true if this comm is using one slice per primitive
   int localRank;
   int localRanks;
   int maxLocalRanks;