code: def __len__(self):
      return 30
  
imports: import copy
  from dataclasses import dataclass, asdict
  import numpy
  import pytest
  import torch
  from baal.active import ActiveLearningDataset
  from baal.utils.pytorch_lightning import ActiveLightningModule, BaalTrainer, ResetCallback, BaaLDataModule
  from pytorch_lightning import LightningModule
  from torch.utils.data import Dataset, DataLoader
  from torchvision.models import vgg16
label_desc: Use image node_id 2 for calling the SimpleDataset obj's underlying member method code with example usage: obj.__len__() and returns: int