Created by: JiayiFeng

In our code, ExecutionContext::HasInput and ExecutionContext::HasOutput only checks whether a name is in an operator's input parameter list:

https://github.com/PaddlePaddle/Paddle/blob/3ff9ba0e6ba1eec282b6e89fb7bea2e2046f01c5/paddle/fluid/framework/operator.h#L193-L197

That's far from enough. We also need to check whether the corresponding variable really exists in the scope.