graph: fix minNchannels for multi-node overwrite (#1230)

src/graph/connect.cc

@@ -674,7 +674,7 @@ ncclResult_t ncclTopoPostset(struct ncclComm* comm, int* firstRanks, int* treePa
 
   int minNchannels = ncclMinNchannels();
   if (comm->nNodes > 1) {
-    minNchannels = std::min(64, maxChannels);
+    minNchannels = std::min(64, minNchannels);
   }
   if (comm->nRanks < 8 && 64 < minNchannels) {
     minNchannels = 2;