[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/entrypoints/test_async_omni_diffusion_config.py

@@ -1,9 +1,13 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+import pytest
+
 from vllm_omni.entrypoints import omni as omni_module
 from vllm_omni.entrypoints.async_omni import AsyncOmni
 
+pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
+
 
 def test_default_stage_config_includes_cache_backend(monkeypatch):
     """Ensure cache_backend/cache_config are preserved in default diffusion stage."""