code: def __getitem__(self, item):
      return self.data[item]
  
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 2 for calling the DatasetWithVaryLength obj's underlying member method code with example usage: obj.__getitem__(item) and returns: self