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/tools/ep_kernels/README.md

@@ -0,0 +1,28 @@
+# Expert parallel kernels
+
+Large-scale cluster-level expert parallel, as described in the [DeepSeek-V3 Technical Report](http://arxiv.org/abs/2412.19437), is an efficient way to deploy sparse MoE models with many experts. However, such deployment requires many components beyond a normal Python package, including system package support and system driver support. It is impossible to bundle all these components into a Python package.
+
+Here we break down the requirements in 2 steps:
+
+1. Build and install the Python libraries ([DeepEP](https://github.com/deepseek-ai/DeepEP)), including necessary dependencies like NVSHMEM. This step does not require any privileged access. Any user can do this.
+2. Configure NVIDIA driver to enable IBGDA. This step requires root access, and must be done on the host machine.
+
+Step 2 is necessary for multi-node deployment.
+
+All scripts accept a positional argument as workspace path for staging the build, defaulting to `$(pwd)/ep_kernels_workspace`.
+
+## Usage
+
+```bash
+# for hopper
+TORCH_CUDA_ARCH_LIST="9.0" bash install_python_libraries.sh
+# for blackwell
+TORCH_CUDA_ARCH_LIST="10.0" bash install_python_libraries.sh
+```
+
+Additional step for multi-node deployment:
+
+```bash
+sudo bash configure_system_drivers.sh # update-initramfs can take several minutes
+sudo reboot # Reboot is required to load the new driver
+```