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/vllm_collective_builder.cuh

@@ -0,0 +1,43 @@
+#pragma once
+
+#include "cutlass/gemm/collective/collective_builder.hpp"
+
+namespace cutlass::gemm::collective {
+using namespace cute;
+
+//
+// VLLMCollectiveBuilder is a wrapper around CollectiveBuilder that allows for
+// for custom kernel tags, allowing you to build custom collectives. Without
+// touching the cutlass library headers, using `CutlassKernelTag` will mean it
+// will resort to using the standard cutlass collective builder.
+//
+
+// Use the default Cutlass collective builder, i.e. use an unmodified cutless
+// collective
+struct CutlassKernelTag {};
+
+template <class KernelTag, class ArchTag, class OpClass, class ElementA,
+          class GmemLayoutA, int AlignmentA, class ElementB, class GmemLayoutB,
+          int AlignmentB, class ElementAccumulator, class TileShape_MNK,
+          class ClusterShape_MNK, class StageCountType,
+          class KernelScheduleType, class Enable = void>
+struct VLLMCollectiveBuilder {
+  static_assert(sizeof(ElementA) == 0,
+                "Could not build a collective for given parameters.");
+};
+
+template <class ArchTag, class OpClass, class ElementA, class GmemLayoutA,
+          int AlignmentA, class ElementB, class GmemLayoutB, int AlignmentB,
+          class ElementAccumulator, class TileShape_MNK, class ClusterShape_MNK,
+          class StageCountType, class KernelScheduleType>
+struct VLLMCollectiveBuilder<
+    CutlassKernelTag, ArchTag, OpClass, ElementA, GmemLayoutA, AlignmentA,
+    ElementB, GmemLayoutB, AlignmentB, ElementAccumulator, TileShape_MNK,
+    ClusterShape_MNK, StageCountType, KernelScheduleType> {
+  using CollectiveOp = typename CollectiveBuilder<
+      ArchTag, OpClass, ElementA, GmemLayoutA, AlignmentA, ElementB,
+      GmemLayoutB, AlignmentB, ElementAccumulator, TileShape_MNK,
+      ClusterShape_MNK, StageCountType, KernelScheduleType>::CollectiveOp;
+};
+
+};  // namespace cutlass::gemm::collective