code: def _write_protobuf(self, filename, nnp):
      with open(filename, "wb") as f:
          f.write(nnp.SerializeToString())
  
file_path: nnabla/python/src/nnabla/utils/converter/nnabla/exporter.py
imports: import glob
  import os
  import shutil
  import tempfile
  import warnings
  import zipfile
  import google.protobuf.text_format
  import numpy
  from nnabla.utils import nnabla_pb2
  from nnabla.logger import logger
label_desc: Use image node_id 3 for calling the NnpExporter obj's underlying member method code with example usage: obj._write_protobuf(filename, nnp) without return types