Mark real multi-device distribution tests in t5gemma2 backbone test

test_distribution, test_distribution_vision, and test_layout_map_mesh_shapes
build an actual ModelParallel/DeviceMesh and need multiple simulated
devices, but were missing @pytest.mark.multi_device. Without it, the
dedicated multi_device CI job (added by keras-team#2809) collects zero tests from
this file -- only test_layout_map_live_presets carried the marker, and
that test is deliberately excluded by the job's own
"not kaggle_key_required and not extra_large" filter.

For test_layout_map_mesh_shapes, the marker is placed as the innermost
decorator (below @parameterized.named_parameters), not outermost:
absl.testing.parameterized generates fresh function objects per expanded
case and only copies pytestmark onto them when it wraps an
already-marked function, not when the mark wraps the parameterized
descriptor itself. Verified empirically that outermost placement causes
pytest to collect 0 of the 18 expanded cases, while innermost placement
collects all 18.

Author: pctablet505

keras_hub/src/models/t5gemma2/t5gemma2_backbone_test.py

@@ -231,6 +231,7 @@ def test_all_presets(self):
                 input_data=self.input_data,
             )
 
+    @pytest.mark.multi_device
     def test_distribution(self):
         # Encoder self_attention and decoder merged_attention share the
         # query/key/value/attention_output naming, so the attention patterns
@@ -279,6 +280,7 @@ def test_distribution(self):
             parity_rtol=1e-4,
         )
 
+    @pytest.mark.multi_device
     def test_distribution_vision(self):
         # Multimodal variant: build a tiny Gemma3VisionEncoder INSIDE the
         # distribution scope via a callable init_kwargs (required -- a vision
@@ -379,6 +381,7 @@ def vision_init_kwargs():
         )
         for shape in CAPPED_MESH_SHAPES
     )
+    @pytest.mark.multi_device
     def test_layout_map_mesh_shapes(self, dims, mesh_shape):
         if keras.backend.backend() != "jax":
             self.skipTest("`ModelParallel` testing requires the Jax backend.")