[Refactor] Move top-level dummy data generation to registry (#32310)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

tests/multimodal/test_processing.py

@@ -926,10 +926,10 @@ def test_limit_mm_per_prompt_dummy(model_id, limit, num_supported, is_valid):
     exc_ctx = nullcontext() if is_valid else pytest.raises(ValueError, match="At most")
 
     with exc_ctx:
-        processor.dummy_inputs.get_decoder_dummy_data(
-            processor,
-            model_config.max_model_len,
+        MULTIMODAL_REGISTRY.get_dummy_mm_inputs(
+            model_config,
             mm_counts=limit_mm_per_prompt,
+            processor=processor,
         )