message: Attach an id for each cost node in virtual_cluster.
diff: @@ -66,6 +66,7 @@ Status VirtualCluster::Run(const GraphDef& graph,
  }
  Costs node_costs;
  + int node_id = 0;
  do {
  OpContext op_context = scheduler.GetCurrNode();
  node_costs = node_estimator_->PredictCosts(op_context);
  @@ -73,6 +74,7 @@ Status VirtualCluster::Run(const GraphDef& graph,
  CostGraphDef::Node* cost_node =
  metadata->mutable_cost_graph()->add_node();
  const string& op_name = op_context.name;
  + cost_node->set_id(node_id++);
  cost_node->set_name(op_name);
  cost_node->set_device(op_context.device_name);
  cost_node->set_compute_cost(
  