class_docstr: Snapshot writer that uses a separate thread.
  
  This class creates a new thread that invokes the actual saving function.
  
  .. seealso::
  
      - :meth:`chainer.training.extensions.snapshot`
code: def create_worker(self, filename, outdir, target, **kwds):
      return threading.Thread(
          target=self.save,
          args=(filename, outdir, target, self._savefun),
          kwargs=self._kwds,
      )
  
imports: import multiprocessing
  import os
  import shutil
  import threading
  from six.moves import queue
  from chainer.serializers import npz
  from chainer import utils
label_desc: Use image node_id 2 for calling the ThreadWriter obj's underlying member method code with example usage: obj.create_worker(filename, outdir, target) and returns: threading