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/cuda_view.cu

@@ -0,0 +1,59 @@
+#include <torch/all.h>
+#include <torch/cuda.h>
+#include <cuda_runtime.h>
+
+// This function assumes that `cpu_tensor` is a CPU tensor,
+// and that UVA (Unified Virtual Addressing) is enabled.
+torch::Tensor get_cuda_view_from_cpu_tensor(torch::Tensor& cpu_tensor) {
+  TORCH_CHECK(cpu_tensor.device().is_cpu(), "Input tensor must be on CPU");
+
+  // handle empty tensor
+  if (cpu_tensor.numel() == 0) {
+    return torch::empty(cpu_tensor.sizes(),
+                        cpu_tensor.options().device(torch::kCUDA));
+  }
+
+  if (cpu_tensor.is_pinned()) {
+    // If CPU tensor is pinned, directly get the device pointer.
+    void* host_ptr = const_cast<void*>(cpu_tensor.data_ptr());
+    void* device_ptr = nullptr;
+    cudaError_t err = cudaHostGetDevicePointer(&device_ptr, host_ptr, 0);
+    TORCH_CHECK(err == cudaSuccess,
+                "cudaHostGetDevicePointer failed: ", cudaGetErrorString(err));
+
+    return torch::from_blob(
+        device_ptr, cpu_tensor.sizes(), cpu_tensor.strides(),
+        [base = cpu_tensor](void*) {},  // keep cpu tensor alive
+        cpu_tensor.options().device(torch::kCUDA));
+  }
+
+  // If CPU tensor is not pinned, allocate a new pinned memory buffer.
+  torch::Tensor contiguous_cpu = cpu_tensor.contiguous();
+  size_t nbytes = contiguous_cpu.nbytes();
+
+  void* host_ptr = nullptr;
+  cudaError_t err = cudaHostAlloc(&host_ptr, nbytes, cudaHostAllocMapped);
+  if (err != cudaSuccess) {
+    AT_ERROR("cudaHostAlloc failed: ", cudaGetErrorString(err));
+  }
+
+  err = cudaMemcpy(host_ptr, contiguous_cpu.data_ptr(), nbytes,
+                   cudaMemcpyDefault);
+  if (err != cudaSuccess) {
+    cudaFreeHost(host_ptr);
+    AT_ERROR("cudaMemcpy failed: ", cudaGetErrorString(err));
+  }
+
+  void* device_ptr = nullptr;
+  err = cudaHostGetDevicePointer(&device_ptr, host_ptr, 0);
+  if (err != cudaSuccess) {
+    cudaFreeHost(host_ptr);
+    AT_ERROR("cudaHostGetDevicePointer failed: ", cudaGetErrorString(err));
+  }
+
+  auto deleter = [host_ptr](void*) { cudaFreeHost(host_ptr); };
+
+  return torch::from_blob(device_ptr, contiguous_cpu.sizes(),
+                          contiguous_cpu.strides(), deleter,
+                          contiguous_cpu.options().device(torch::kCUDA));
+}