tmpfix(tokenizer_config): force `GPT2TokenizerFast`

tokenizer_config.json

@@ -306,6 +306,6 @@
   "eos_token": "<|endoftext|>",
   "model_max_length": 1000000000000000019884624838656,
   "pad_token": "<|endoftext|>",
-  "tokenizer_class": "GPT2Tokenizer",
+  "tokenizer_class": "GPT2TokenizerFast",
   "unk_token": "<|endoftext|>"
 }