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/.buildkite/test_areas/pytorch.yaml

@@ -0,0 +1,57 @@
+group: PyTorch
+depends_on: 
+  - image-build
+steps:
+- label: PyTorch Compilation Unit Tests
+  timeout_in_minutes: 10
+  source_file_dependencies:
+    - vllm/
+    - tests/compile
+  commands:
+  # Run unit tests defined directly under compile/,
+  # not including subdirectories, which are usually heavier
+  # tests covered elsewhere.
+  # Use `find` to launch multiple instances of pytest so that
+  # they do not suffer from https://github.com/vllm-project/vllm/issues/28965
+  # However, find does not normally propagate error codes, so we combine it with xargs
+  # (using -0 for proper path handling)
+  - "find compile/ -maxdepth 1 -name 'test_*.py' -print0 | xargs -0 -n1 -I{} pytest -s -v '{}'"
+
+- label: PyTorch Compilation Passes Unit Tests
+  timeout_in_minutes: 20
+  source_file_dependencies:
+    - vllm/
+    - tests/compile/passes
+  commands:
+  - pytest -s -v compile/passes --ignore compile/passes/distributed
+
+- label: PyTorch Fullgraph Smoke Test
+  timeout_in_minutes: 35
+  source_file_dependencies:
+  - vllm/
+  - tests/compile
+  commands:
+  # Run smoke tests under fullgraph directory, except test_full_graph.py
+  # as it is a heavy test that is covered in other steps.
+  # Use `find` to launch multiple instances of pytest so that
+  # they do not suffer from https://github.com/vllm-project/vllm/issues/28965
+  - "find compile/fullgraph/ -name 'test_*.py' -not -name 'test_full_graph.py' -exec pytest -s -v {} \\;"
+
+- label: PyTorch Fullgraph
+  timeout_in_minutes: 30
+  source_file_dependencies:
+  - vllm/
+  - tests/compile
+  commands:
+    # fp8 kv scales not supported on sm89, tested on Blackwell instead
+  - pytest -v -s compile/fullgraph/test_full_graph.py -k 'not test_fp8_kv_scale_compile'
+
+- label: Pytorch Nightly Dependency Override Check # 2min
+  # if this test fails, it means the nightly torch version is not compatible with some
+  # of the dependencies. Please check the error message and add the package to whitelist
+  # in /vllm/tools/pre_commit/generate_nightly_torch_test.py
+  soft_fail: true
+  source_file_dependencies:
+  - requirements/nightly_torch_test.txt
+  commands:
+  - bash standalone_tests/pytorch_nightly_dependency.sh