Fit google C++ style

Make code more clearer

* Clone method will create a new object instance, which is as same as
  itself.
* This is the first step to remove shared_ptr for OperatorBase

* insert rnn's backward into Backward()

* add device_context into backward_test

* make stepnet member

* add pybind support

* fix Inputs Outputs

* remove unique_ptr

This reverts commit fb61512c.