Add missing config value (num_hidden_layers)

This is present in the 256M model: https://huggingface.co/HuggingFaceTB/SmolVLM2-256M-Video-Instruct/blob/main/config.json#L24

config.json

@@ -20,6 +20,7 @@
     "model_type": "llama",
     "neftune_noise_alpha": 0.0,
     "num_attention_heads": 15,
+    "num_hidden_layers": 32,
     "num_key_value_heads": 5,
     "pad_token_id": 2,
     "perceiver_config": {