Fix tokenizer_class from TokenizersBackend to PreTrainedTokenizerFast for vLLM compat

tokenizer_config.json

@@ -1,14 +1,14 @@
 {
-  "backend": "tokenizers",
-  "bos_token": "<|startoftext|>",
-  "clean_up_tokenization_spaces": false,
-  "eos_token": "<|return|>",
-  "is_local": false,
-  "model_input_names": [
-    "input_ids",
-    "attention_mask"
-  ],
-  "model_max_length": 1000000000000000019884624838656,
-  "pad_token": "<|endoftext|>",
-  "tokenizer_class": "TokenizersBackend"
+    "backend": "tokenizers",
+    "bos_token": "<|startoftext|>",
+    "clean_up_tokenization_spaces": false,
+    "eos_token": "<|return|>",
+    "is_local": false,
+    "model_input_names": [
+        "input_ids",
+        "attention_mask"
+    ],
+    "model_max_length": 131072,
+    "pad_token": "<|endoftext|>",
+    "tokenizer_class": "PreTrainedTokenizerFast"
 }