make check_graph choosable (#13674)

test=develop

paddle/fluid/framework/parallel_executor.cc

@@ -156,11 +156,13 @@ ParallelExecutor::ParallelExecutor(
                            params, member_->local_scopes_, member_->use_cuda_);
 #endif
 
+  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) {
     member_->executor_.reset(new details::ThreadedSSAGraphExecutor(