Rail optimization for rings (#1140)

- Modifies the ring creation algorithm to be friendlier to rail-optimized topologies (should not affect classic fabric topologies)

[ROCm/rccl commit: 4cb62f999a]

projects/rccl/tools/topo_expl/utils.cpp

@@ -823,6 +823,29 @@ ncclResult_t initTransportsRank_1(struct ncclComm* comm, struct allGatherInfo *a
   // Determine local Nvls support
   //NCCLCHECK(ncclNvlsInit(comm));
 
+  // [RCCL] Compute hostIdx (based on hostHash)
+  {
+    comm->topo->nHosts = 0;
+    for (int r = 0; r < nranks; r++) {
+      int isNewHost = 1;
+      // Check if this is the first time this hostname has been used
+      for (int i = 0; i < r && isNewHost; i++) {
+        if (comm->peerInfo[i].hostHash == comm->peerInfo[r].hostHash)
+          isNewHost = 0;
+      }
+      if (isNewHost)
+      {
+        // Check if this is the same hostname associated with this rank
+        if (comm->peerInfo[r].hostHash == comm->peerInfo[rank].hostHash)
+        {
+          comm->topo->hostIdx = comm->topo->nHosts;
+          printf("Rank %d is on host %d\n", rank, comm->topo->hostIdx);
+        }
+        comm->topo->nHosts++;
+      }
+    }
+  }
+
   // Get rings and trees
   ringGraph.id = 0;
   ringGraph.pattern = NCCL_TOPO_PATTERN_RING;