update switch kernel documentation (#7597)
update switch kernel documentation (#7597)

* update switch_kernel.md

* update the demo code

* update

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