class_docstr: A dataset wrapping over a Numpy binary (.npy, .npz) file.
  
  If the file is a .npy file, then a single numpy array is loaded.
  If the file is a .npz file with multiple arrays, then a list of
  numpy arrays are loaded, ordered by their key in the archive.
  
  Sparse matrix is not yet supported.
  
  Parameters
  ----------
  filename : str
      Path to the .npy or .npz file.
  kwargs
      Keyword arguments are passed to np.load.
  
  Properties
  ----------
  keys: list of str or None
      The list of keys loaded from the .npz file.
code: def keys(self):
      return self._keys
  
imports: import io
  import os
  import gc
  import glob
  import pickle
  import warnings
  import multiprocessing
  from functools import partial
  import numpy
  from mxnet import context, npx
  from mxnet.gluon.data import ArrayDataset, SimpleDataset
  from mxnet.gluon.data.dataloader import ForkingPickler, _as_in_context, default_mp_batchify_fn, default_batchify_fn
label_desc: Use image node_id 2 for calling the NumpyDataset obj's underlying member method code with example usage: obj.keys() and returns: self