[Refactor 1/N] Centralize Omni config resolution (#5140)

Signed-off-by: Chenguang ZHENG <645327136@qq.com>

tests/diffusion/test_diffusion_config_propagation.py

@@ -131,7 +131,7 @@ class TestCreateDefaultDiffusion:
 
     def test_dtype_serialized_as_string(self):
         stages = StageConfigFactory.create_default_diffusion({"dtype": torch.float16, "model": "x"})
-        assert stages[0]["engine_args"]["dtype"] == "torch.float16"
+        assert stages[0]["engine_args"]["dtype"] == "float16"
 
     def test_cache_backend_defaults_to_none(self):
         stages = StageConfigFactory.create_default_diffusion({"model": "x"})