add some comments for sparse table
add some comments for sparse table

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