tmpfix(tokenizer_config): force `GPT2TokenizerFast`

tokenizer_config.json

@@ -311,6 +311,6 @@
   "pad_token": "<|endoftext|>",
   "pad_token_type_id": 0,
   "padding_side": "left",
-  "tokenizer_class": "GPT2Tokenizer",
+  "tokenizer_class": "GPT2TokenizerFast",
   "unk_token": "<|endoftext|>"
 }