code: def test_add_seq_len(self):
      ds = DataSet({"x": [[1, 2], [2, 3, 4], [3]], "y": [4, 5, 6]})
      ds.add_seq_len("x")
      print(ds)
  
imports: import os
  import pytest
  import numpy
  from fastNLP.core.dataset import DataSet, FieldArray, Instance, ApplyResultException
  from fastNLP import logger
label_desc: Use image node_id 31 for calling the TestDataSetMethods obj's underlying member method code with example usage: obj.test_add_seq_len() without return types