[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/cache/test_cache_backends.py

@@ -22,6 +22,8 @@ from vllm_omni.diffusion.cache.selector import get_cache_backend
 from vllm_omni.diffusion.cache.teacache.backend import TeaCacheBackend
 from vllm_omni.diffusion.data import DiffusionCacheConfig
 
+pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
+
 
 class TestCacheDiTBackend:
     """Test CacheDiTBackend implementation."""