[CI] Reorganize the L1 L2 use cases and add markers (#2449)

Signed-off-by: zhumingjue <zhumingjue@huawei.com>

tests/test_diffusion_config_propagation.py

@@ -7,6 +7,7 @@ Regression tests for https://github.com/vllm-project/vllm-omni/issues/1862
 
 from collections.abc import Mapping
 
+import pytest
 import torch
 
 from vllm_omni.config.stage_config import StageConfigFactory
@@ -15,6 +16,8 @@ from vllm_omni.diffusion.data import (
     OmniDiffusionConfig,
 )
 
+pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
+
 
 def _roundtrip_diffusion_config(**kwargs) -> OmniDiffusionConfig:
     """Simulate the real path: create_default_diffusion → OmniDiffusionConfig.