[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/worker/test_omni_gpu_model_runner.py

@@ -1,10 +1,13 @@
 from contextlib import contextmanager
 from types import SimpleNamespace
 
+import pytest
 import torch
 
 from vllm_omni.worker.gpu_model_runner import OmniGPUModelRunner
 
+pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
+
 
 class DummyBuffer:
     """A minimal buffer wrapper that exposes the `.gpu` attribute."""