tmpfix(tokenizer_config): force `GPT2TokenizerFast`

tokenizer_config.json

@@ -38,6 +38,6 @@
   "bos_token": "<|endoftext|>",
   "clean_up_tokenization_spaces": true,
   "eos_token": "<|endoftext|>",
-  "tokenizer_class": "GPT2Tokenizer",
+  "tokenizer_class": "GPT2TokenizerFast",
   "unk_token": "<|endoftext|>"
 }