NCCL_TREES variable and rome model fixes (#856)

tools/topo_expl/utils.cpp

@@ -1019,7 +1019,7 @@ ncclResult_t initTransportsRank_3(struct ncclComm* comm, struct allGatherInfo *a
   NCCLCHECKGOTO(ncclCalloc(&rings, nranks*MAXCHANNELS), ret, fail);
 
   NCCLCHECKGOTO(ncclTopoPostset(comm, nodesFirstRank, nodesTreePatterns, allTopoRanks, rings, graphs, nc), ret, fail);
-  if (comm->topo->pivotA2ANumBiRings == 3) NCCLCHECK(ncclTreeBasePostset(comm, &treeGraph));
+  if (comm->topo->treeDefined) NCCLCHECK(ncclTreeBasePostset(comm, &treeGraph));
 
   // AllGather3 - end