Adding doc for sums layer (#6857)
Adding doc for sums layer (#6857)


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