Update Readers Python API
Update Readers Python API

1. Combine 'open_files', 'multi_pass_reader' and 'threaded_reader'
together to make the new 'open_files' interface.
2. Add some docstring.
3. Simplify interface names of 'create_XXX_reader', e.g, rename
'create_double_buffer_reader' to 'double_buffer'.

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