[bugfix] [ROCm] Fix premature CUDA initialization in platform detection (#33941)

Signed-off-by: Kourosh Hakhamaneshi <kourosh@anyscale.com>

tests/cuda/test_platform_no_cuda_init.py

@@ -0,0 +1,48 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""
+Test that platform imports do not prematurely initialize CUDA.
+
+This is critical for Ray-based multi-GPU setups where workers need to
+set CUDA_VISIBLE_DEVICES after importing vLLM but before CUDA is initialized.
+If CUDA is initialized during import, device_count() gets locked and ignores
+subsequent env var changes.
+"""
+
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+SCRIPTS_DIR = Path(__file__).parent / "scripts"
+
+
+def run_script(script_name: str) -> subprocess.CompletedProcess:
+    """Run a test script in a subprocess with clean CUDA state."""
+    script_path = SCRIPTS_DIR / script_name
+    return subprocess.run(
+        [sys.executable, str(script_path)],
+        capture_output=True,
+        text=True,
+    )
+
+
+def test_platform_import_does_not_init_cuda():
+    """Test that importing vllm.platforms does not initialize CUDA."""
+    result = run_script("check_platform_no_cuda_init.py")
+    if result.returncode != 0:
+        pytest.fail(f"Platform import initialized CUDA:\n{result.stderr}")
+
+
+def test_device_count_respects_env_after_platform_import():
+    """Test that device_count respects CUDA_VISIBLE_DEVICES after import."""
+    result = run_script("check_device_count_respects_env.py")
+    if result.returncode != 0:
+        pytest.fail(
+            f"device_count does not respect env var after import:\n{result.stderr}"
+        )
+
+
+if __name__ == "__main__":
+    pytest.main([__file__, "-v"])