code: def __len__(self):
      return len(self._outputs)
  
imports: from typing import Iterator, List
  import torch
  import pytest
  import json
  from allennlp.common.testing import AllenNlpTestCase
  from allennlp.common import Params
  from allennlp.common.util import sanitize
  from allennlp.data.data_loaders import TensorDict
  from allennlp.evaluation import Serializer
  from allennlp.evaluation.serializers import SimpleSerializer
label_desc: Use image node_id 3 for calling the DummyDataLoader obj's underlying member method code with example usage: obj.__len__() and returns: len