code: def __len__(self):
      return len(self.data)
  
file_path: fastNLP/tests/core/samplers/test_unrepeated_sampler.py
imports: from itertools import chain
  import pytest
  from fastNLP.core.samplers import UnrepeatedRandomSampler, UnrepeatedSortedSampler, UnrepeatedSequentialSampler
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