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/use_existing_torch.py

@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import argparse
+import glob
+import sys
+
+# Only strip targeted libraries when checking prefix
+TORCH_LIB_PREFIXES = (
+    # requirements/*.txt/in
+    "torch=",
+    "torchvision=",
+    "torchaudio=",
+    # pyproject.toml
+    '"torch =',
+    '"torchvision =',
+    '"torchaudio =',
+)
+
+
+def main(argv):
+    parser = argparse.ArgumentParser(
+        description="Strip torch lib requirements to use installed version."
+    )
+    parser.add_argument(
+        "--prefix",
+        action="store_true",
+        help="Strip prefix matches only (default: False)",
+    )
+    args = parser.parse_args(argv)
+
+    for file in (
+        *glob.glob("requirements/*.txt"),
+        *glob.glob("requirements/*.in"),
+        "pyproject.toml",
+    ):
+        with open(file) as f:
+            lines = f.readlines()
+        if "torch" in "".join(lines).lower():
+            with open(file, "w") as f:
+                for line in lines:
+                    if (
+                        args.prefix
+                        and not line.lower().strip().startswith(TORCH_LIB_PREFIXES)
+                        or not args.prefix
+                        and "torch" not in line.lower()
+                    ):
+                        f.write(line)
+                    else:
+                        print(f">>> removed from {file}:", line.strip())
+
+
+if __name__ == "__main__":
+    main(sys.argv[1:])