code: def test_start_token(self):
      with self.assertRaises(RuntimeError):
          create_agent(
              {
                  "model": "hugging_face/gpt2",
                  "add_special_tokens": False,
                  "add_start_token": True,
              }
          )
  
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 2 for calling the TestGpt2 obj's underlying member method code with example usage: obj.test_start_token() without return types