* update NCE_cn.rst, PRelu_cn.rst, to_variable_cn.rst, test=develop

* fix NCE, PRelu, to_variable, test=develop

* add comment for class Layer, test=develop

* add comment for dygraph layer, test=develop

* add comment for dygraph layer, test=develop

* add comment for name, test=develop

* update to_variable_cn.rst, test=develop