Add head_dim for vLLM compatibility

config.json

@@ -42,5 +42,6 @@
   "use_cache": false,
   "use_grouped_mm": false,
   "use_qk_norm": false,
-  "vocab_size": 151552
-}
+  "vocab_size": 151552,
+  "head_dim": 64
+}