Use valid class name for tokenizer

tokenizer_config.json

@@ -4,6 +4,6 @@
   "clean_up_tokenization_spaces": true,
   "eos_token": "<|endoftext|>",
   "model_max_length": 1000000000000000019884624838656,
-  "tokenizer_class": "GPTNeoXTokenizer",
+  "tokenizer_class": "GPTNeoXTokenizerFast",
   "unk_token": "<|endoftext|>"
 }