[Test] Isolate CPU collective workers from the host GPU platform

Signed-off-by: tzhouam <tzhouam@connect.ust.hk>

tests/helpers/cpu_collective_server.py

@@ -0,0 +1,19 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM-Omni project
+"""Import preload for the CPU-only collective forkserver.
+
+This module is imported in the forkserver process, before it starts workers.
+Its CUDA visibility does not alter the pytest process or CUDA parity workers.
+"""
+
+import os
+
+os.environ["CUDA_VISIBLE_DEVICES"] = ""
+
+import torch  # noqa: E402
+
+# Retain the expensive worker-module imports across CPU process groups.
+from tests.diffusion.distributed import test_comm, test_pipeline_parallel  # noqa: E402, F401
+
+if torch.cuda.is_initialized():
+    raise RuntimeError("CPU collective forkserver must not initialize CUDA")