fix ProgramToGraph
fix ProgramToGraph

when while_grad, it writes multiple @EMPTY@ with no VarDesc.

luotao1/Paddle,chengduoZH/Paddle,reyoung/Paddle,PaddlePaddle/Paddle,chengduoZH/Paddle,baidu/Paddle,chengduoZH/Paddle,QiJune/Paddle,QiJune/Paddle,chengduoZH/Paddle,QiJune/Paddle,baidu/Paddle,tensor-tang/Paddle,luotao1/Paddle,QiJune/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,reyoung/Paddle,baidu/Paddle,luotao1/Paddle,luotao1/Paddle,reyoung/Paddle,luotao1/Paddle,reyoung/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,luotao1/Paddle,chengduoZH/Paddle,baidu/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,QiJune/Paddle,PaddlePaddle/Paddle,reyoung/Paddle,QiJune/Paddle,reyoung/Paddle,baidu/Paddle