code: def close(self):
      if not self._closed:
          for cf in self._cache_prefetchers:
              cf.close()
          self._closed = True
  
imports: import atexit
  import csv
  import os
  import threading
  from collections import OrderedDict, deque
  from time import sleep
  import numpy
  from nnabla.config import nnabla_config
  from nnabla.logger import logger
  from nnabla.utils.communicator_util import current_communicator
  from six.moves import queue
  from .data_source import DataSource
  from .data_source_loader import FileReader, load
label_desc: Use image node_id 3 for calling the CacheReaderWithPrefetch obj's underlying member method code with example usage: obj.close() without return types