Updated `hidden_act` to `silu`

Hi!

Sergio from the HF team here.
Same problem discussed here: https://huggingface.co/mistralai/Mistral-Small-3.1-24B-Instruct-2503/discussions/74

config.json

@@ -30,7 +30,7 @@
   "vision_config": {
     "attention_dropout": 0.0,
     "head_dim": 64,
-    "hidden_act": "gelu",
+    "hidden_act": "silu",
     "hidden_size": 1024,
     "image_size": 1540,
     "initializer_range": 0.02,