Created by: luotao1

• Similar to remove_op method, this PR add remove_var(var_name) method for removing a particular varible.
• refine test_protobuf_desc.py with https://github.com/PaddlePaddle/Paddle/pull/9600#discussion_r178780837
• refine block_desc.h and protobuf.cc when git commit and cpplint fails:

paddle/fluid/pybind/protobuf.cc:101:  Is this a non-const reference? If so, make const or use a pointer: T &self  

[runtime/references] [2]
paddle/fluid/pybind/protobuf.cc:110:  Is this a non-const reference? If so, make const or use a pointer: py::module &m  

[runtime/references] [2]
paddle/fluid/pybind/protobuf.cc:154:  Is this a non-const reference? If so, make const or use a pointer: py::module &m  

[runtime/references] [2]
paddle/fluid/pybind/protobuf.cc:215:  Is this a non-const reference? If so, make const or use a pointer: py::module &m  

[runtime/references] [2]
paddle/fluid/pybind/protobuf.cc:266:  Is this a non-const reference? If so, make const or use a pointer: py::module &m  

[runtime/references] [2]
paddle/fluid/pybind/protobuf.cc:130:  Add #include <tuple> for tuple<>  [build/include_what_you_use] [4]
paddle/fluid/pybind/protobuf.cc:304:  Add #include <string> for string  [build/include_what_you_use] [4]
Done processing paddle/fluid/pybind/protobuf.cc