add design doc on keys of operaror kernel type (#6782)
add design doc on keys of operaror kernel type (#6782)

* add design doc on keys of operator kernel type

* follow comments

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