Migrate weak_ref_tensor to sgl-kernel (#12505)

sgl-kernel/csrc/memory/weak_ref_tensor.cpp

@@ -0,0 +1,35 @@
+/* Copyright 2025 SGLang Team. All Rights Reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+==============================================================================*/
+
+// Adapted from: https://github.com/vllm-project/vllm/blob/main/csrc/ops.h
+
+#include <ATen/ATen.h>
+#include <ATen/Tensor.h>
+
+#include <vector>
+
+at::Tensor weak_ref_tensor(const at::Tensor& tensor) {
+  TORCH_CHECK(tensor.is_cuda(), "weak_ref_tensor expects a CUDA tensor");
+
+  void* data_ptr = tensor.data_ptr();
+  std::vector<int64_t> sizes = tensor.sizes().vec();
+  std::vector<int64_t> strides = tensor.strides().vec();
+
+  auto options = tensor.options();
+
+  auto new_tensor = at::from_blob(data_ptr, sizes, strides, options);
+
+  return new_tensor;
+}