code: def launch_thread(self):
      self._pool = multiprocessing.Pool(
          processes=self.n_processes,
          initializer=_fetch_setup,
          initargs=(self.dataset, self.mem_size, self.mem_bulk),
          maxtasksperchild=self.maxtasksperchild,
      )
      if self._interruption_testing:
          pids = self._pool.map(_report_pid, range(self.n_processes))
          print(" ".join(map(str, pids)))
          sys.stdout.flush()
  
      thread = threading.Thread(target=self._run, name="prefetch_loop")
      thread.setDaemon(True)
      thread.start()
      self._thread = thread
      return thread
  
imports: from __future__ import division
  import datetime
  import multiprocessing
  from multiprocessing import sharedctypes
  import signal
  import sys
  import threading
  import warnings
  import numpy
  import six
  from chainer.dataset import iterator
  from chainer.iterators import _statemachine
  from chainer.iterators.order_samplers import ShuffleOrderSampler
label_desc: Use image node_id 7 for calling the _PrefetchLoop obj's underlying member method code with example usage: obj.launch_thread() and returns: thread