message: Also print the function def before partitioning, when v=2.
diff: @@ -700,6 +700,9 @@ Status ProcessFunctionLibraryRuntime::InstantiateMultiDevice(
  options.graph_collector->CollectOptimizedGraph(def);
  }
  + VLOG(2) << "Main function graph to be partitioned:";
  + VLOG(2) << DebugString(graph->ToGraphDefDebug());
  +
  std::unordered_map<string, std::unique_ptr<Graph>> subgraphs;
  TF_RETURN_IF_ERROR(
  PartitionFunctionGraph(device_set_, std::move(graph), &subgraphs));
  