code: def test_add_field(self):
      dd = DataSet()
      dd.add_field("x", [[1, 2, 3]] * 10)
      dd.add_field("y", [[1, 2, 3, 4]] * 10)
      dd.add_field("z", [[5, 6]] * 10)
      assert len(dd) == 10
      assert dd.field_arrays["x"].content == [[1, 2, 3]] * 10
      assert dd.field_arrays["y"].content == [[1, 2, 3, 4]] * 10
      assert dd.field_arrays["z"].content == [[5, 6]] * 10
  
      with pytest.raises(RuntimeError):
          dd.add_field("??", [[1, 2]] * 40)
  
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 2 for calling the TestDataSetMethods obj's underlying member method code with example usage: obj.test_add_field() without return types