code: def test_model_loading(self):
      for (
          pretrained_model_name
      ) in GPT2Tokenizer.available_checkpoints():
          tokenizer = GPT2Tokenizer(
              pretrained_model_name=pretrained_model_name
          )
          _ = tokenizer.map_text_to_token(
              "Munich and Berlin are nice cities"
          )
  
file_path: texar/tests/data/tokenizers/gpt2_tokenizer_test.py
imports: import json
  import os
  import pickle
  import tempfile
  import tensorflow
  from texar.tf.data.tokenizers.gpt2_tokenizer import GPT2Tokenizer
  from texar.tf.utils.test import pretrained_test
label_desc: Use image node_id 3 for calling the GPT2TokenizerTest obj's underlying member method code with example usage: obj.test_model_loading() without return types