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/quantization/machete/machete_collective_builder.cuh

@@ -0,0 +1,31 @@
+#pragma once
+
+#include "cutlass_extensions/vllm_collective_builder.cuh"
+#include "machete_mainloop.cuh"
+
+namespace cutlass::gemm::collective {
+using namespace cute;
+
+struct MacheteKernelTag {};
+
+template <class ElementPairA_, class GmemLayoutA_, int AlignmentA,
+          class ElementPairB_, class GmemLayoutB_, int AlignmentB,
+          class ElementAccumulator, class TileShape_MNK, class ClusterShape_MNK,
+          class StageCountType, class KernelScheduleType>
+struct VLLMCollectiveBuilder<
+    MacheteKernelTag, arch::Sm90, arch::OpClassTensorOp, ElementPairA_,
+    GmemLayoutA_, AlignmentA, ElementPairB_, GmemLayoutB_, AlignmentB,
+    ElementAccumulator, TileShape_MNK, ClusterShape_MNK, StageCountType,
+    KernelScheduleType,
+    cute::enable_if_t<(
+        cute::is_same_v<KernelScheduleType, KernelTmaWarpSpecialized> ||
+        cute::is_same_v<KernelScheduleType, KernelTmaWarpSpecializedPingpong> ||
+        cute::is_same_v<KernelScheduleType,
+                        KernelTmaWarpSpecializedCooperative>)>> {
+  using CollectiveOp = machete::MacheteCollectiveMma<
+      ElementPairA_, GmemLayoutA_, AlignmentA, ElementPairB_, GmemLayoutB_,
+      AlignmentB, ElementAccumulator, TileShape_MNK, ClusterShape_MNK,
+      StageCountType, KernelScheduleType>;
+};
+
+};  // namespace cutlass::gemm::collective