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/concat_mla_q.cuh

@@ -0,0 +1,60 @@
+#ifndef CONCAT_MLA_Q_CUH_
+#define CONCAT_MLA_Q_CUH_
+
+#include <cuda_bf16.h>
+#include <cuda_fp16.h>
+
+#include "cuda_vec_utils.cuh"
+
+namespace vllm {
+
+// Concatenates ql_nope [num_tokens, num_heads, NOPE_DIM] and
+// q_pe [num_tokens, num_heads, 64]
+// into q_out [num_tokens, num_heads, NOPE_DIM+64].
+// Currently instantiated only for NOPE_DIM=512.
+// Rope dim is hardcoded to 64 (DeepSeek V3.2 MLA)
+template <typename DType, int NOPE_DIM>
+__global__ void ConcatMLAQKernel(
+    DType* __restrict__ q_out, const DType* __restrict__ ql_nope,
+    const DType* __restrict__ q_pe, const int num_tokens, const int num_heads,
+    const int64_t out_stride_0, const int64_t out_stride_1,
+    const int64_t nope_stride_0, const int64_t nope_stride_1,
+    const int64_t pe_stride_0, const int64_t pe_stride_1) {
+  const int flat_warp_id = (blockIdx.x * blockDim.x + threadIdx.x) >> 5;
+  if (flat_warp_id >= num_tokens * num_heads) return;
+
+  const int token_id = flat_warp_id / num_heads;
+  const int head_id = flat_warp_id % num_heads;
+  const int lane_id = threadIdx.x & 31;
+
+  constexpr bool use_256b = VLLM_256B_PTX_ENABLED;
+  constexpr int nope_vec_loads =
+      NOPE_DIM * sizeof(DType) / (VecTraits<use_256b>::ARCH_MAX_VEC_SIZE * 32);
+
+  const DType* nope_src =
+      ql_nope + token_id * nope_stride_0 + head_id * nope_stride_1;
+  DType* nope_dst = q_out + token_id * out_stride_0 + head_id * out_stride_1;
+
+#pragma unroll
+  for (int i = 0; i < nope_vec_loads; i++) {
+    const int offset = i * 32 + lane_id;
+    if constexpr (use_256b) {
+      st256_cs(reinterpret_cast<u32x8_t*>(nope_dst) + offset,
+               ld256_cs(reinterpret_cast<const u32x8_t*>(nope_src) + offset));
+    } else {
+      st128_cs(reinterpret_cast<int4*>(nope_dst) + offset,
+               ld128_cs(reinterpret_cast<const int4*>(nope_src) + offset));
+    }
+  }
+
+  const int* rope_src = reinterpret_cast<const int*>(
+      q_pe + token_id * pe_stride_0 + head_id * pe_stride_1);
+  int* rope_dst = reinterpret_cast<int*>(q_out + token_id * out_stride_0 +
+                                         head_id * out_stride_1 + NOPE_DIM);
+
+  st32_cs(rope_dst + lane_id, ld32_cs(rope_src + lane_id));
+}
+
+}  // namespace vllm
+
+#endif  // CONCAT_MLA_Q_CUH_