code: def test_apply_cannot_modify_instance(self):
      ds = DataSet({"x": [[1, 2, 3, 4]] * 40, "y": [[5, 6]] * 40})
  
      def modify_inplace(instance):
          instance["words"] = 1
  
      ds.apply(modify_inplace)
  
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 17 for calling the TestDataSetMethods obj's underlying member method code with example usage: obj.test_apply_cannot_modify_instance() without return types