Unit Tests for testing channels (#1222)

src/graph/connect.cc

@@ -674,7 +674,11 @@ ncclResult_t ncclTopoPostset(struct ncclComm* comm, int* firstRanks, int* treePa
 
   int minNchannels = ncclMinNchannels();
   if (comm->nNodes > 1) {
-    minNchannels = std::min(64, minNchannels);
+    minNchannels = std::min(64, maxChannels);
+  }
+  if (comm->nRanks < 8 && 64 < minNchannels) {
+    minNchannels = 2;
+    WARN("NCCL_MIN_NCHANNELS set by environment is ignored due to less than 8 GPUs.");
   }
 
   if (mscclEnabled() && (comm->topo->mscclEnabled || mscclForceEnabled())) {