Add grad_op_maker for sequence_pool.
Add grad_op_maker for sequence_pool.

Canpio/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,luotao1/Paddle,tensor-tang/Paddle,putcn/Paddle,Canpio/Paddle,pkuyym/Paddle,tensor-tang/Paddle,PaddlePaddle/Paddle,pkuyym/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,Canpio/Paddle,Canpio/Paddle,baidu/Paddle,pkuyym/Paddle,putcn/Paddle,Canpio/Paddle,tensor-tang/Paddle,lcy-seso/Paddle,reyoung/Paddle,jacquesqiao/Paddle,luotao1/Paddle,Canpio/Paddle,putcn/Paddle,tensor-tang/Paddle,pkuyym/Paddle,Canpio/Paddle,chengduoZH/Paddle,luotao1/Paddle,lcy-seso/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,baidu/Paddle,jacquesqiao/Paddle,chengduoZH/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,baidu/Paddle,lcy-seso/Paddle,pkuyym/Paddle,reyoung/Paddle,putcn/Paddle,reyoung/Paddle,chengduoZH/Paddle,baidu/Paddle,PaddlePaddle/Paddle,QiJune/Paddle,jacquesqiao/Paddle,jacquesqiao/Paddle,QiJune/Paddle,QiJune/Paddle,QiJune/Paddle,luotao1/Paddle,Canpio/Paddle,QiJune/Paddle,reyoung/Paddle,pkuyym/Paddle,baidu/Paddle,lcy-seso/Paddle,PaddlePaddle/Paddle,putcn/Paddle,lcy-seso/Paddle,QiJune/Paddle,putcn/Paddle,chengduoZH/Paddle,reyoung/Paddle,chengduoZH/Paddle,lcy-seso/Paddle,reyoung/Paddle,jacquesqiao/Paddle,jacquesqiao/Paddle