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

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

tests/cuda/scripts/check_device_count_respects_env.py

@@ -0,0 +1,23 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""Check that device_count respects CUDA_VISIBLE_DEVICES after platform import."""
+
+import os
+import sys
+
+for key in ["CUDA_VISIBLE_DEVICES", "HIP_VISIBLE_DEVICES", "ROCR_VISIBLE_DEVICES"]:
+    os.environ.pop(key, None)
+
+import torch  # noqa: E402
+
+from vllm.platforms import current_platform  # noqa: F401, E402
+
+os.environ["CUDA_VISIBLE_DEVICES"] = "0"
+count = torch.cuda.device_count()
+
+if count == 0:
+    sys.exit(0)  # Skip: no GPUs available
+
+assert count == 1, f"device_count()={count}, expected 1"
+print("OK")