code: def __len__(self):
      return len(self._y)
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import pytest
  import numpy
  import torch
  import torch.nn
  import torch.optim
  from torch.utils.data import Dataset, DataLoader
  import bluefog.torch
  from bluefog.common import topology_util
label_desc: Use image node_id 2 for calling the SimpleDataset obj's underlying member method code with example usage: obj.__len__() and returns: len