code: def test_custom_special_tokens(self):
      from parlai.agents.hugging_face.dict import Gpt2DictionaryAgent
      from parlai.core.params import ParlaiParser
  
      parser = ParlaiParser(False, False)
      parser.set_defaults(
          gpt2_size="small", add_special_tokens=True, fp16=True
      )
      Gpt2DictionaryAgent.add_cmdline_args(parser, partial_opt=None)
      with testing_utils.tempdir() as tmpdir:
          opt = parser.parse_kwargs(
              dict_file=os.path.join(tmpdir, "dict")
          )
          dict_agent = Gpt2DictionaryAgent(opt)
          oldtokens = dict_agent.txt2vec("Hi VOLDEMORT")
          prevlen = len(dict_agent)
          dict_agent.add_additional_special_tokens(["VOLDEMORT"])
          newlen = len(dict_agent)
          assert newlen == prevlen + 1
          tokens = dict_agent.txt2vec("Hi VOLDEMORT")
          assert tokens != oldtokens
          assert len(tokens) < len(oldtokens)
  
imports: import unittest
  from parlai.core.agents import create_agent
  import parlai.utils.testing
  import parlai.scripts.multiprocessing_train
  import parlai.scripts.build_dict
  import os
  import copy
label_desc: Use image node_id 1 for calling the TestHuggingFaceDict obj's underlying member method code with example usage: obj.test_custom_special_tokens() without return types