Design doc of SelectedRows (#4652)
Design doc of SelectedRows (#4652)

* Design doc of SelectedRows

* Follow comments

* Update protobuf message

* Follow comments, seperate LoDTensorDesc and SelectedRows Desc

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