Fix #3582

Fit Google C++ Style

* add_op could take a unique_ptr or a const reference. If unique_ptr is
  taken, the NetOp will take care of that operator's life cycle. If a
  const reference is taken, that op will be Cloned.

* Also simplify pybind implementation by using OperatorBase as holder
  type.

Fit google C++ style

* 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