* PyBind and SWIG of paddle cannot be load in a single Python process,
lazy import all SWIG library of Paddle. Otherwise, the glog, gflags
are imported twice in a same Python process.
* Note that all PyBind11 return C++ std::string as an unicode. For
protobuf, it is need be cast to `str` before use them.
* Add unit test for Get `OpProtos`