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/standalone_tests/lazy_imports.py

@@ -0,0 +1,22 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+# Description: Test the lazy import module
+# The utility function cannot be placed in `vllm.utils`
+# this needs to be a standalone script
+import sys
+
+# List of modules that should not be imported too early.
+# Lazy import `torch._inductor.async_compile` to avoid creating
+# too many processes before we set the number of compiler threads.
+# Lazy import `cv2` to avoid bothering users who only use text models.
+# `cv2` can easily mess up the environment.
+module_names = ["torch._inductor.async_compile", "cv2"]
+
+# set all modules in `module_names` to be None.
+# if we import any modules during `import vllm`, there would be a
+# hard error and nice stacktrace on the first import.
+for module_name in module_names:
+    sys.modules[module_name] = None  # type: ignore[assignment]
+
+import vllm  # noqa