code: def __len__(self):
      return 50
  
imports: import os
  import numpy
  import pytest
  from PIL import Image
  from torch import nn
  from torch import optim
  from torch.utils.data import Dataset
  from torchvision.models import vgg
  from torchvision.transforms import ToTensor, Resize, Compose
  from baal.active import ActiveLearningDataset
  from baal.active import ActiveLearningLoop
  from baal.active import heuristics
  from baal.modelwrapper import ModelWrapper
  from baal.calibration import DirichletCalibrator
label_desc: Use image node_id 2 for calling the DummyDataset obj's underlying member method code with example usage: obj.__len__() and returns: int