code: def __len__(self):
      return self.length
  
imports: import bisect
  from unittest import TestCase
  from unittest.mock import patch
  import numpy
  from torch.utils.data import ConcatDataset, Dataset
  from mmdet.datasets.samplers import GroupMultiSourceSampler, MultiSourceSampler
label_desc: Use image node_id 2 for calling the DummyDataset obj's underlying member method code with example usage: obj.__len__() and returns: self