Add feature_extractor config for audio support

processor_config.json

@@ -28,5 +28,15 @@
     }
   },
   "image_seq_length": 280,
-  "processor_class": "Gemma4Processor"
-}
+  "processor_class": "Gemma4Processor",
+  "feature_extractor": {
+    "feature_extractor_type": "Gemma4AudioFeatureExtractor",
+    "sampling_rate": 16000,
+    "num_mel_filters": 128,
+    "fft_length": 512,
+    "hop_length": 160,
+    "chunk_duration": 8.0,
+    "overlap_duration": 1.0
+  },
+  "audio_ms_per_token": 40
+}