fix RNN and IfElse syntax in Block design (#4210)
fix RNN and IfElse syntax in Block design (#4210)

* fix block syntax

* add builder

* fix argument names

* uniform all Var to var

* fix c demo

* factor set_outputs

* update block

* delete as_ifelse_input interface from block design

* simplify set_outputs and output_num

* make the c++ codes compatible with demo

* fix compatible

* fix syntax

* update ie

* update syntax

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