Add head_dim field for vLLM compatibility

config.json

@@ -38,5 +38,6 @@
   "use_cache": false,
   "use_grouped_mm": true,
   "use_qk_norm": false,
-  "vocab_size": 151552
-}
+  "vocab_size": 151552,
+  "head_dim": 64
+}