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/docker/Dockerfile.tpu

@@ -0,0 +1,36 @@
+ARG NIGHTLY_DATE="20250730"
+ARG BASE_IMAGE="us-central1-docker.pkg.dev/tpu-pytorch-releases/docker/xla:nightly_3.12_tpuvm_$NIGHTLY_DATE"
+
+FROM $BASE_IMAGE
+WORKDIR /workspace/vllm
+
+# Install some basic utilities
+RUN apt-get update && apt-get install -y \
+    git \
+    ffmpeg libsm6 libxext6 libgl1 && \
+    rm -rf /var/lib/apt/lists/*
+
+# Build vLLM.
+COPY . .
+ARG GIT_REPO_CHECK=0
+RUN --mount=type=bind,source=.git,target=.git \
+    if [ "$GIT_REPO_CHECK" != 0 ]; then bash tools/check_repo.sh; fi
+
+# Remove existing versions of dependencies
+# TODO: These packages will remain as dead weight in the Docker image layers.
+# We should find a way to build the image without uninstalling these.
+# Consider using a different base image.
+RUN pip uninstall -y torch torch_xla torchvision
+
+ENV VLLM_TARGET_DEVICE="tpu"
+RUN --mount=type=cache,target=/root/.cache/pip \
+    --mount=type=bind,source=.git,target=.git \
+    python3 -m pip install \
+        -r requirements/tpu.txt
+
+RUN --mount=type=cache,target=/root/.cache/pip python3 -m pip install -e .
+
+# install development dependencies (for testing)
+RUN --mount=type=cache,target=/root/.cache/pip python3 -m pip install -e tests/vllm_test_utils
+
+CMD ["/bin/bash"]