delete useless codes in softmax backward.
delete useless codes in softmax backward.

reyoung/Paddle,lispc/Paddle,yu239/Paddle,lispc/Paddle,yu239/Paddle,hedaoyuan/Paddle,tensor-tang/Paddle,pengli09/Paddle,PaddlePaddle/Paddle,Canpio/Paddle,luotao1/Paddle,lispc/Paddle,putcn/Paddle,pengli09/Paddle,pengli09/Paddle,luotao1/Paddle,hedaoyuan/Paddle,Canpio/Paddle,pengli09/Paddle,luotao1/Paddle,jacquesqiao/Paddle,lispc/Paddle,lispc/Paddle,lispc/Paddle,luotao1/Paddle,Canpio/Paddle,putcn/Paddle,pkuyym/Paddle,pkuyym/Paddle,baidu/Paddle,yu239/Paddle,yu239/Paddle,hedaoyuan/Paddle,tensor-tang/Paddle,luotao1/Paddle,Canpio/Paddle,QiJune/Paddle,QiJune/Paddle,QiJune/Paddle,Canpio/Paddle,jacquesqiao/Paddle,reyoung/Paddle,pengli09/Paddle,lcy-seso/Paddle,pkuyym/Paddle,pkuyym/Paddle,yu239/Paddle,tensor-tang/Paddle,lcy-seso/Paddle,lispc/Paddle,yu239/Paddle,PaddlePaddle/Paddle,reyoung/Paddle,Canpio/Paddle,pkuyym/Paddle,chengduoZH/Paddle,QiJune/Paddle,reyoung/Paddle,hedaoyuan/Paddle,pengli09/Paddle,baidu/Paddle,lcy-seso/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,QiJune/Paddle,yu239/Paddle,jacquesqiao/Paddle,baidu/Paddle,QiJune/Paddle,pkuyym/Paddle,lcy-seso/Paddle,chengduoZH/Paddle,hedaoyuan/Paddle,lispc/Paddle,lcy-seso/Paddle,hedaoyuan/Paddle,chengduoZH/Paddle,yu239/Paddle,PaddlePaddle/Paddle,hedaoyuan/Paddle,baidu/Paddle,chengduoZH/Paddle,reyoung/Paddle,lcy-seso/Paddle,PaddlePaddle/Paddle,reyoung/Paddle,baidu/Paddle,PaddlePaddle/Paddle,putcn/Paddle,Canpio/Paddle,putcn/Paddle,jacquesqiao/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,tensor-tang/Paddle,putcn/Paddle,pengli09/Paddle,jacquesqiao/Paddle,pengli09/Paddle,putcn/Paddle,jacquesqiao/Paddle,luotao1/Paddle,Canpio/Paddle,chengduoZH/Paddle,hedaoyuan/Paddle