code: def __init__(self, num_of_data=100):
      self.data = np.arange(num_of_data)
  
imports: import numpy
  import pytest
  from itertools import chain
  from copy import deepcopy
  from array import array
  from tests.helpers.datasets.normal_data import NormalSampler, NormalBatchSampler
  from fastNLP.core.samplers import ReproduceBatchSampler, BucketedBatchSampler, RandomBatchSampler
label_desc: Use image node_id 1 to create a new DatasetWithVaryLength object  with example: obj = DatasetWithVaryLength(num_of_data)