code: def __len__(self):
      return len(self.data)
  
imports: import numpy
  import pytest
  from functools import partial
  from itertools import chain
  from copy import deepcopy
  from fastNLP.core.samplers.reproducible_sampler import RandomSampler, SortedSampler, SequentialSampler
  from tests.helpers.datasets.torch_data import TorchNormalDataset
label_desc: Use image node_id 3 for calling the DatasetWithVaryLength obj's underlying member method code with example usage: obj.__len__() and returns: len