class_docstr: Abstract dataset class. All datasets should have this interface.
  
  Subclasses need to override `__getitem__`, which returns the i-th
  element, and `__len__`, which returns the total number elements.
  
  .. note:: An mxnet or numpy array can be directly used as a dataset.
code: def __getitem__(self, idx):
      raise NotImplementedError
  
imports: import os
  from ...None import recordio, ndarray
  from ...util import default_array
label_desc: Use image node_id 1 for calling the Dataset obj's underlying member method code with example usage: obj.__getitem__(idx) without return types