Optimize Gemma4 small decoder test config to reduce memory usage

Author: lukebaumann

tests/unit/nnx_decoders_test.py

@@ -796,6 +796,8 @@ def test_gemma4_small_decoder(self):
             "base_mlp_dim=256",
             "dtype=float32",
             "weight_dtype=float32",
+            "hidden_size_per_layer_input=128",
+            "vocab_size_per_layer_input=256",
             "vocab_size=256",
         ],
         override_model_config=True,