Avoid copying a GraphDef and GrapplerItem in AnalyticalCostEstimator::PredictCosts().

PredictCosts() is most often called from VirtualCluster::Run(), which has two special properties:

1. The `optimized_graph` argument is the same as `this->item_->graph`.
2. The `costs` output parameter is ignored.

Optimizing for this case avoids unnecessary GraphDef copies, which can be expensive at startup time.

PiperOrigin-RevId: 261120012

tensorflow/core/grappler/clusters/virtual_cluster.cc

@@ -85,9 +85,8 @@ Status VirtualCluster::Run(const GrapplerItem& item, RunMetadata* metadata) {
   }
 
   TF_RETURN_IF_ERROR(estimator_->Initialize(item));
-  Costs ignored_costs;
   TF_RETURN_IF_ERROR(
-      estimator_->PredictCosts(item.graph, metadata, &ignored_costs));
+      estimator_->PredictCosts(item.graph, metadata, /*cost=*/nullptr));
 
   const std::unordered_map<string, DeviceProperties>& device = GetDevices();
   std::unordered_map<string, int64> peak_mem_usage =

tensorflow/core/grappler/costs/analytical_cost_estimator.cc

@@ -149,12 +149,24 @@ Status AnalyticalCostEstimator::Initialize(const GrapplerItem& item) {
 Status AnalyticalCostEstimator::PredictCosts(const GraphDef& optimized_graph,
                                              RunMetadata* run_metadata,
                                              Costs* costs) const {
-  GraphDef graph_copy = optimized_graph;
-  GrapplerItem item = item_->WithGraph(std::move(graph_copy));
+  std::unique_ptr<GrapplerItem> item_storage;
+  const GrapplerItem* item;
+  // Many callers to PredictCosts() pass the same optimized_graph as was used
+  // to initialize the estimator.
+  if (&optimized_graph == &item_->graph) {
+    item = item_;
+  } else {
+    GraphDef graph_copy = optimized_graph;
+    item_storage = absl::make_unique<GrapplerItem>(
+        item_->WithGraph(std::move(graph_copy)));
+    item = item_storage.get();
+  }
 
-  auto status = scheduler_->Init(&item);
+  auto status = scheduler_->Init(item);
   if (!status.ok()) {
-    costs->execution_time = Costs::Duration::max();
+    if (costs) {
+      costs->execution_time = Costs::Duration::max();
+    }
     return status;
   }
 
@@ -203,7 +215,11 @@ Status AnalyticalCostEstimator::PredictCosts(const GraphDef& optimized_graph,
   }
 
   // run_metadata gets step_stats and partition_graphs from Summary.
-  *costs = scheduler_->Summary(run_metadata);
+  if (costs) {
+    *costs = scheduler_->Summary(run_metadata);
+  } else if (run_metadata) {
+    scheduler_->GenerateRunMetadata(run_metadata);
+  }
 
   if (VLOG_IS_ON(1)) {
     bool verbose = VLOG_IS_ON(2);