[Bugfix] Allow CUDA_VISIBLE_DEVICES='' in Platform.device_id_to_physical_device_id (#18979)

Signed-off-by: Seiji Eicher <seiji@anyscale.com>

tests/config/test_config_generation.py

@@ -0,0 +1,38 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import pytest
+
+from vllm.engine.arg_utils import EngineArgs
+from vllm.model_executor.layers.quantization.quark.utils import deep_compare
+
+
+def test_cuda_empty_vs_unset_configs(monkeypatch: pytest.MonkeyPatch):
+    """Test that configs created with normal (untouched) CUDA_VISIBLE_DEVICES
+    and CUDA_VISIBLE_DEVICES="" are equivalent. This ensures consistent
+    behavior regardless of whether GPU visibility is disabled via empty string
+    or left in its normal state.
+    """
+
+    def create_config():
+        engine_args = EngineArgs(model="deepseek-ai/DeepSeek-V2-Lite",
+                                 trust_remote_code=True)
+        return engine_args.create_engine_config()
+
+    # Create config with CUDA_VISIBLE_DEVICES set normally
+    normal_config = create_config()
+
+    # Create config with CUDA_VISIBLE_DEVICES=""
+    with monkeypatch.context() as m:
+        m.setenv("CUDA_VISIBLE_DEVICES", "")
+        empty_config = create_config()
+
+    normal_config_dict = vars(normal_config)
+    empty_config_dict = vars(empty_config)
+
+    # Remove instance_id before comparison as it's expected to be different
+    normal_config_dict.pop("instance_id", None)
+    empty_config_dict.pop("instance_id", None)
+
+    assert deep_compare(normal_config_dict, empty_config_dict), (
+        "Configs with normal CUDA_VISIBLE_DEVICES and CUDA_VISIBLE_DEVICES=\"\""
+        " should be equivalent")