test: bound CPU vocoder reference threads (#8347)

Signed-off-by: andyluo7 <andy.luo@amd.com>

tests/helpers/fixtures/cpu_threads.py

@@ -0,0 +1,52 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM-Omni project
+
+from __future__ import annotations
+
+import time
+from collections.abc import Callable, Generator
+from typing import Protocol
+
+import pytest
+
+
+class _TorchThreadAPI(Protocol):
+    def get_num_threads(self) -> int: ...
+
+    def set_num_threads(self, num_threads: int) -> None: ...
+
+
+def _single_threaded_cpu(
+    request: pytest.FixtureRequest,
+    load_torch: Callable[[], _TorchThreadAPI],
+) -> Generator[None, None, None]:
+    """Implement the marker guard separately so its state handling is testable."""
+    if request.node.get_closest_marker("cpu") is None:
+        yield
+        return
+
+    torch = load_torch()
+    previous_threads = torch.get_num_threads()
+    started = time.perf_counter()
+    try:
+        torch.set_num_threads(1)
+        print(f"{request.node.nodeid}: CPU reference threads {previous_threads} -> {torch.get_num_threads()}")
+        yield
+    finally:
+        torch.set_num_threads(previous_threads)
+        print(
+            f"{request.node.nodeid}: CPU reference elapsed {time.perf_counter() - started:.3f}s; "
+            f"restored threads {torch.get_num_threads()}"
+        )
+
+
+@pytest.fixture
+def single_threaded_cpu(request: pytest.FixtureRequest) -> Generator[None, None, None]:
+    """Bound opted-in CPU tests while leaving GPU-marked cases unchanged."""
+
+    def load_torch() -> _TorchThreadAPI:
+        import torch
+
+        return torch
+
+    yield from _single_threaded_cpu(request, load_torch)