* add cudnn compatiable rnn cell api for dygraph

* update sample code

* update some typos

* fix specify name in param_attr problem

* add pre-commit check

* remove duplicate import, test=develop

* add unittest coverage, test=develop

* make code more tight, test=develop

* cudnn_compatibale -> use_cudnn_impl, test=develop

* change api name, test=develop