Fix duplicated forward/backward in trainer.
Fix duplicated forward/backward in trainer.

QiJune/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,QiJune/Paddle,wen-bo-yang/Paddle,jacquesqiao/Paddle,lispc/Paddle,wen-bo-yang/Paddle,Canpio/Paddle,tensor-tang/Paddle,lcy-seso/Paddle,lispc/Paddle,hedaoyuan/Paddle,lcy-seso/Paddle,Canpio/Paddle,yu239/Paddle,Canpio/Paddle,cxysteven/Paddle,putcn/Paddle,Canpio/Paddle,jacquesqiao/Paddle,tensor-tang/Paddle,baidu/Paddle,luotao1/Paddle,pkuyym/Paddle,livc/Paddle,PaddlePaddle/Paddle,lcy-seso/Paddle,livc/Paddle,cxysteven/Paddle,PaddlePaddle/Paddle,gangliao/Paddle,jacquesqiao/Paddle,hedaoyuan/Paddle,chengduoZH/Paddle,yu239/Paddle,gangliao/Paddle,emailweixu/Paddle,gangliao/Paddle,livc/Paddle,yu239/Paddle,cxysteven/Paddle,reyoung/Paddle,emailweixu/Paddle,reyoung/Paddle,cxysteven/Paddle,jacquesqiao/Paddle,pengli09/Paddle,chengduoZH/Paddle,baidu/Paddle,putcn/Paddle,luotao1/Paddle,hedaoyuan/Paddle,gangliao/Paddle,wen-bo-yang/Paddle,Canpio/Paddle,lcy-seso/Paddle,pengli09/Paddle,reyoung/Paddle,pkuyym/Paddle,luotao1/Paddle,lcy-seso/Paddle,lispc/Paddle,pengli09/Paddle,emailweixu/Paddle,lispc/Paddle,putcn/Paddle,hedaoyuan/Paddle,hedaoyuan/Paddle,baidu/Paddle,QiJune/Paddle,yu239/Paddle,luotao1/Paddle,lispc/Paddle,wen-bo-yang/Paddle,gangliao/Paddle,tensor-tang/Paddle,emailweixu/Paddle,QiJune/Paddle,tensor-tang/Paddle,wen-bo-yang/Paddle,chengduoZH/Paddle,PaddlePaddle/Paddle,pengli09/Paddle,chengduoZH/Paddle,yu239/Paddle,lcy-seso/Paddle,pengli09/Paddle,luotao1/Paddle,yu239/Paddle,pkuyym/Paddle,chengduoZH/Paddle,cxysteven/Paddle,pengli09/Paddle,pengli09/Paddle,gangliao/Paddle,Canpio/Paddle,emailweixu/Paddle,wen-bo-yang/Paddle,reyoung/Paddle,hedaoyuan/Paddle,gangliao/Paddle,yu239/Paddle,putcn/Paddle,luotao1/Paddle,pengli09/Paddle,pkuyym/Paddle,cxysteven/Paddle,yu239/Paddle,PaddlePaddle/Paddle,pkuyym/Paddle,putcn/Paddle,baidu/Paddle,lispc/Paddle,jacquesqiao/Paddle,baidu/Paddle,QiJune/Paddle,Canpio/Paddle,livc/Paddle,livc/Paddle,emailweixu/Paddle,Canpio/Paddle,wen-bo-yang/Paddle,emailweixu/Paddle,livc/Paddle,pkuyym/Paddle,reyoung/Paddle,reyoung/Paddle,hedaoyuan/Paddle,livc/Paddle,PaddlePaddle/Paddle,lispc/Paddle,jacquesqiao/Paddle,lispc/Paddle,hedaoyuan/Paddle,PaddlePaddle/Paddle,putcn/Paddle,cxysteven/Paddle,QiJune/Paddle