code: def __len__(self):
      return 20
  
imports: import unittest
  from unittest.mock import Mock
  import numpy
  import pytest
  import torch
  from torch import nn
  from torch.utils.data import Dataset
  from baal.calibration import DirichletCalibrator
  from baal.modelwrapper import ModelWrapper
label_desc: Use image node_id 1 for calling the DummyDataset obj's underlying member method code with example usage: obj.__len__() and returns: int