Add vLLM v0.18.1 source tree with KV transfer abort fix

third_party/vllm/ now tracked in git for direct patch management.
Based on vLLM v0.18.1 release with one patch applied:

  vllm/v1/core/sched/scheduler.py:
    Replace fatal assert with graceful skip when KV transfer callback
    arrives for an already-aborted request during PD disaggregated serving.

Future vLLM modifications should be made directly in third_party/vllm/
and committed normally. The patches/ directory is kept as documentation
of what changed from upstream.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

third_party/vllm/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.accelerator.device_count()
+
+if count == 0:
+    sys.exit(0)  # Skip: no GPUs available
+
+assert count == 1, f"device_count()={count}, expected 1"
+print("OK")

third_party/vllm/tests/cuda/scripts/check_platform_no_cuda_init.py

@@ -0,0 +1,20 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""Check that vllm.platforms import does not initialize CUDA."""
+
+import os
+
+for key in ["CUDA_VISIBLE_DEVICES", "HIP_VISIBLE_DEVICES", "ROCR_VISIBLE_DEVICES"]:
+    os.environ.pop(key, None)
+
+import torch  # noqa: E402
+
+assert not torch.cuda.is_initialized(), "CUDA initialized before import"
+
+from vllm.platforms import current_platform  # noqa: E402
+
+assert not torch.cuda.is_initialized(), (
+    f"CUDA was initialized during vllm.platforms import on {current_platform}"
+)
+print("OK")