code: def __len__(self):
      return self.num_examples
  
imports: import functools
  import os
  import unittest
  from unittest.mock import MagicMock
  import torch
  from mmf.datasets.base_dataset import BaseDataset
  from mmf.datasets.mmf_dataset_builder import MMFDatasetBuilder
  from mmf.utils.general import get_current_device
  from omegaconf import OmegaConf
label_desc: Use image node_id 3 for calling the SimpleMMFDataset obj's underlying member method code with example usage: obj.__len__() and returns: self