make check_graph choosable (#13674)

test=develop

paddle/fluid/framework/parallel_executor.cc

@@ -156,10 +156,12 @@ ParallelExecutor::ParallelExecutor(
                            params, member_->local_scopes_, member_->use_cuda_);
 #endif
 
-  // If the loss_var_name is given, the number of graph should be only one.
-  if (loss_var_name.size()) {
-    PADDLE_ENFORCE_EQ(ir::GraphNum(*graph), 1,
-                      "The number of graph should be only one");
+  if (VLOG_IS_ON(5)) {
+    // If the loss_var_name is given, the number of graph should be only one.
+    if (loss_var_name.size()) {
+      PADDLE_ENFORCE_EQ(ir::GraphNum(*graph), 1,
+                        "The number of graph should be only one");
+    }
   }
 
   if (exec_strategy.type_ == ExecutionStrategy::kDefault) {