speed up while_op

paddle/fluid/operators/while_op.cc

@@ -57,12 +57,12 @@ class WhileOp : public framework::OperatorBase {
 
     PADDLE_ENFORCE(platform::is_cpu_place(cond.place()),
                    "Condition of while op must in CPU memory.");
+
+    auto ctx = executor.Prepare(*program, block->ID());
     while (cond.data<bool>()[0]) {
       auto &current_scope = scope.NewScope();
       step_scopes->push_back(&current_scope);
-
-      executor.Run(*program, &current_scope, block->ID(),
-                   false /*create_local_scope*/);
+      executor.RunPreparedContext(ctx.get(), &current_scope, false);
     }
   }
 };
@@ -109,6 +109,7 @@ class WhileGradOp : public framework::OperatorBase {
     framework::Executor executor(dev_place);
     auto *block = Attr<framework::BlockDesc *>(kStepBlock);
     auto *program = block->Program();
+    auto ctx = executor.Prepare(*program, block->ID());
 
     auto *step_scopes =
         scope.FindVar(Input(kStepScopes))->GetMutable<StepScopeVar>();
@@ -161,8 +162,7 @@ class WhileGradOp : public framework::OperatorBase {
           }
         }
       }
-
-      executor.Run(*program, *cur_scope_iter, block->ID(), false);
+      executor.RunPreparedContext(ctx.get(), *cur_scope_iter, false);
 
       auto &pg_names = Outputs(kXGRAD);
       auto &p_names = Inputs(kX);