[CI] Add pytest markers to current tests and update the doc. (#577)

Signed-off-by: Alicia <115451386+congw729@users.noreply.github.com>
Co-authored-by: Hongsheng Liu <liuhongsheng4@huawei.com>

tests/diffusion/test_diffusion_worker.py

@@ -17,6 +17,8 @@ import torch
 
 from vllm_omni.diffusion.worker.diffusion_worker import DiffusionWorker
 
+pytestmark = [pytest.mark.core_model, pytest.mark.diffusion, pytest.mark.cpu]
+
 
 @pytest.fixture
 def mock_od_config():