Created by: carlushuang

in https://github.com/PaddlePaddle/Paddle/blob/develop/benchmark/fluid/fluid_benchmark.py#L182

  build_strategy.fuse_broadcast_op = args.fuse_broadcast_op

however fuse_broadcast_op of BuildStrategy is not exported by pybind, hence may result in crash when run fluid_benchmark.py

need to fix this by exporting fuse_broadcast_op in pybind.cc