Add hidden_act for Candle compatibility

config.json

@@ -51,5 +51,6 @@
   "sparse_pred_ignore_index": -100,
   "sparse_prediction": false,
   "transformers_version": "4.57.3",
-  "vocab_size": 50368
-}
+  "vocab_size": 50368,
+  "hidden_act": "gelu"
+}