Rename model_class to the split generation class

kf_config.json

@@ -2,7 +2,7 @@
   "library_name": "kerasformers",
   "kerasformers_version": "1.2.1",
   "model_module": "kerasformers.models.gemma4",
-  "model_class": "Gemma4Generate",
+  "model_class": "Gemma4ConditionalGenerate",
   "variant": "gemma-4-e2b-it",
   "weights": "model.weights.json",
   "schema_version": 2,
@@ -112,4 +112,4 @@
   "audio_token_id": 258881,
   "pad_token_id": 0,
   "use_bidirectional_vision": false
-}
+}