[slimtensor] Add aoti_torch_copy_ for SlimTensor

Add SlimTensor-based `aoti_torch_copy_()` - Copies data from source tensor to destination tensor. Delegates to SlimTensor's `copy_()` which handles all device combinations (CPU-CPU, CPU-CUDA, CUDA-CPU, CUDA-CUDA).

Differential Revision: [D90126246](https://our.internmc.facebook.com/intern/diff/D90126246/)

[ghstack-poisoned]

Author: Gasoonjia

backends/cuda/runtime/shims/memory_slim.cpp

@@ -186,6 +186,26 @@ AOTITorchError aoti_torch__reinterpret_tensor(
   return Error::Ok;
 }
 
+AOTITorchError
+aoti_torch_copy_(Tensor* self, Tensor* src, int32_t non_blocking) {
+  (void)non_blocking; // SlimTensor::copy_() is always synchronous for now
+
+  ET_CHECK_OR_RETURN_ERROR(
+      self != nullptr, InvalidArgument, "aoti_torch_copy_: self is null");
+
+  ET_CHECK_OR_RETURN_ERROR(
+      src != nullptr, InvalidArgument, "aoti_torch_copy_: src is null");
+
+  // SlimTensor::copy_() handles:
+  // - Same numel validation
+  // - Same dtype validation
+  // - CPU-CPU, CPU-CUDA, CUDA-CPU, CUDA-CUDA copies
+  // - Contiguous fast path and non-contiguous element-wise copy
+  self->copy_(*src);
+
+  return Error::Ok;
+}
+
 } // extern "C"
 
 } // namespace executorch::backends::cuda

backends/cuda/runtime/shims/memory_slim.h

@@ -128,6 +128,21 @@ AOTI_SHIM_EXPORT AOTITorchError aoti_torch__reinterpret_tensor(
     int64_t storage_offset,
     Tensor** ret_new_tensor);
 
+/**
+ * Copies data from source tensor to destination tensor.
+ *
+ * Handles all device combinations (CPU-CPU, CPU-CUDA, CUDA-CPU, CUDA-CUDA)
+ * and supports tensors with different strides. The destination tensor must
+ * already be allocated with sufficient storage.
+ *
+ * @param self Destination tensor (must not be null)
+ * @param src Source tensor to copy from (must not be null)
+ * @param non_blocking If true, the copy may be asynchronous (currently ignored)
+ * @return AOTITorchError error code (Error::Ok on success)
+ */
+AOTI_SHIM_EXPORT AOTITorchError
+aoti_torch_copy_(Tensor* self, Tensor* src, int32_t non_blocking);
+
 } // extern "C"
 
 } // namespace executorch::backends::cuda

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

@@ -76,3 +76,4 @@ def define_common_targets():
     cuda_shim_slim_cpp_unittest("aoti_torch_delete_tensor_object")
     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_")