code: def test_train_split_len(self):
      self.assertEqual(len(self.train), 70)
      self.assertEqual(len(self.val), 20)
      self.assertEqual(len(self.test), 10)
  
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 2 for calling the TestMMFDatasetBuilder obj's underlying member method code with example usage: obj.test_train_split_len() without return types