code: def test_fold(self):
      assert DatatypeHelper.fold("train") == "train"
      assert DatatypeHelper.fold("train:ordered") == "train"
      assert DatatypeHelper.fold("train:stream") == "train"
      assert DatatypeHelper.fold("train:stream:ordered") == "train"
      assert DatatypeHelper.fold("train:evalmode") == "train"
      assert DatatypeHelper.fold("train:stream:evalmode") == "train"
  
      assert DatatypeHelper.fold("valid") == "valid"
      assert DatatypeHelper.fold("valid:stream") == "valid"
  
      assert DatatypeHelper.fold("test") == "test"
      assert DatatypeHelper.fold("test:stream") == "test"
  
imports: from parlai.core.opt import Opt
  from parlai.utils.misc import Timer, round_sigfigs, set_namedtuple_defaults, nice_report
  import parlai.utils.strings
  from copy import deepcopy
  import random
  import time
  import unittest
  from parlai.utils.data import DatatypeHelper
  from parlai.utils.curated_response import generate_init_prompot, generate_safe_response
label_desc: Use image node_id 1 for calling the TestDatatypeHelper obj's underlying member method code with example usage: obj.test_fold() without return types