Expose RMSProp optimizer. (#9247)
Expose RMSProp optimizer. (#9247)

* Add RMSProp optimizer warpper.

* Follow comments.

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