code: def __len__(self):
      return len(self.datasets[0])
  
imports: from unittest import TestCase
  import operator
  import pytest
  import torch
  from torch.utils.data import TensorDataset, DataLoader, Dataset
  from torch import nn
  import torchmetrics
  from bigdl.nano.pytorch import Trainer
  from bigdl.nano.utils.common import compare_version
label_desc: Use image node_id 3 for calling the ChainTensorDataset obj's underlying member method code with example usage: obj.__len__() and returns: len