[slimtensor] Add aoti_torch_assign_tensors_out for SlimTensor

Add SlimTensor-based `aoti_torch_assign_tensors_out()` - Creates a new tensor handle that moves the ownership of storage from the source tensor to ret tensor via SharedPtr.

Differential Revision: [D90126243](https://our.internmc.facebook.com/intern/diff/D90126243/)

[ghstack-poisoned]

Author: Gasoonjia

backends/cuda/runtime/shims/memory_slim.cpp

@@ -206,6 +206,25 @@ aoti_torch_copy_(Tensor* self, Tensor* src, int32_t non_blocking) {
   return Error::Ok;
 }
 
+AOTITorchError aoti_torch_assign_tensors_out(Tensor* src, Tensor** ret_dst) {
+  ET_CHECK_OR_RETURN_ERROR(
+      src != nullptr,
+      InvalidArgument,
+      "aoti_torch_assign_tensors_out: src is null");
+
+  ET_CHECK_OR_RETURN_ERROR(
+      ret_dst != nullptr,
+      InvalidArgument,
+      "aoti_torch_assign_tensors_out: ret_dst is null");
+
+  // Move the source tensor into the destination. After this operation,
+  // the source tensor will be left in an undefined state (reset).
+  // This differs from aoti_torch_new_tensor_handle which copies the tensor.
+  *ret_dst = new Tensor(std::move(*src));
+
+  return Error::Ok;
+}
+
 } // extern "C"
 
 } // namespace executorch::backends::cuda

backends/cuda/runtime/shims/memory_slim.h

@@ -143,6 +143,20 @@ AOTI_SHIM_EXPORT AOTITorchError aoti_torch__reinterpret_tensor(
 AOTI_SHIM_EXPORT AOTITorchError
 aoti_torch_copy_(Tensor* self, Tensor* src, int32_t non_blocking);
 
+/**
+ * Moves a tensor into a new handle and assigns it to the output parameter.
+ *
+ * Unlike aoti_torch_new_tensor_handle which copies, this function moves the
+ * source tensor into the destination. After this operation, the source tensor
+ * is left in an undefined/reset state and should not be used.
+ *
+ * @param src Source tensor to move from (must not be null, will be reset)
+ * @param ret_dst Output parameter for the new tensor handle
+ * @return AOTITorchError error code (Error::Ok on success)
+ */
+AOTI_SHIM_EXPORT AOTITorchError
+aoti_torch_assign_tensors_out(Tensor* src, Tensor** ret_dst);
+
 } // extern "C"
 
 } // namespace executorch::backends::cuda

backends/cuda/runtime/shims/tests/targets.bzl

@@ -77,3 +77,4 @@ def define_common_targets():
     cuda_shim_slim_cpp_unittest("aoti_torch_new_tensor_handle")
     cuda_shim_slim_cpp_unittest("aoti_torch__reinterpret_tensor")
     cuda_shim_slim_cpp_unittest("aoti_torch_copy_")
+    cuda_shim_slim_cpp_unittest("aoti_torch_assign_tensors_out")