Update Gemma3 model example with 1B decoder.

PiperOrigin-RevId: 748439985

Author: haozha111

ai_edge_torch/generative/examples/gemma3/gemma3.py

@@ -154,7 +154,7 @@ def forward(
 def get_fake_model_config(**kwargs) -> Gemma3MMConfig:
   return Gemma3MMConfig(
       image_encoder_config=image_encoder.get_fake_image_encoder_config(),
-      decoder_config=decoder.get_fake_decoder_config_4b(**kwargs),
+      decoder_config=decoder.get_fake_decoder_config_1b(**kwargs),
       image_token_id=127,
       image_projection_scale=128**0.5,
       image_projection_use_bias=False,