code: def test_delete_instance(self):
      dd = DataSet()
      old_length = 2
      dd.add_field("x", [[1, 2, 3]] * old_length)
      dd.add_field("y", [[1, 2, 3, 4]] * old_length)
      dd.delete_instance(0)
      assert len(dd) == old_length - 1
      dd.delete_instance(0)
      assert len(dd) == old_length - 2
  
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 4 for calling the TestDataSetMethods obj's underlying member method code with example usage: obj.test_delete_instance() without return types