code: def setUp(self, mock):
      self.length_a = 100
      self.dataset_a = DummyDataset(self.length_a, flag="a")
      self.length_b = 1000
      self.dataset_b = DummyDataset(self.length_b, flag="b")
      self.dataset = DummyConcatDataset(
          [self.dataset_a, self.dataset_b]
      )
  
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 1 for calling the TestMultiSourceSampler obj's underlying member method code with example usage: obj.setUp(mock) without return types