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/csrc/cutlass_extensions/common.cpp

@@ -0,0 +1,11 @@
+#include "cutlass_extensions/common.hpp"
+
+int32_t get_sm_version_num() {
+  int32_t major_capability, minor_capability;
+  cudaDeviceGetAttribute(&major_capability, cudaDevAttrComputeCapabilityMajor,
+                         0);
+  cudaDeviceGetAttribute(&minor_capability, cudaDevAttrComputeCapabilityMinor,
+                         0);
+  int32_t version_num = major_capability * 10 + minor_capability;
+  return version_num;
+}