[ET-VK][EZ] Assign trivially moveable values to const-ref variables

The [`toType()` methods return a reference](https://github.com/pytorch/executorch/blob/be618c27ee75fb949f3479115f4ea585ea291451/backends/vulkan/runtime/graph/containers/Value.h#L222) so there's no need to copy the return value.

Differential Revision: [D55701570](https://our.internmc.facebook.com/intern/diff/D55701570/)

ghstack-source-id: 221145473
Pull Request resolved: pytorch/executorch#2834

Author: jorgep31415

backends/vulkan/runtime/graph/ops/PrepackNode.cpp

@@ -36,8 +36,8 @@ void PrepackNode::encode(ComputeGraph* graph) {
   api::Context* const context = graph->context();
   api::PipelineBarrier pipeline_barrier{};
 
-  TensorRef tref = graph->get_val(tref_).toTensorRef();
-  vTensor packed = graph->get_val(packed_).toTensor();
+  TensorRef& tref = graph->get_val(tref_).toTensorRef();
+  vTensor& packed = graph->get_val(packed_).toTensor();
 
   size_t numel = api::utils::multiply_integers(tref.sizes);
   api::StorageBuffer staging(graph->context(), tref.dtype, numel);

backends/vulkan/runtime/graph/ops/impl/Staging.cpp

@@ -65,7 +65,7 @@ ValueRef prepack(
     const api::GPUMemoryLayout layout) {
   TensorRef& tref = graph.get_val(vref).toTensorRef();
   ValueRef v = graph.add_tensor(tref.sizes, tref.dtype, layout);
-  vTensor t = graph.get_val(v).toTensor();
+  vTensor& t = graph.get_val(v).toTensor();
 
   api::ShaderInfo shader = get_nchw_to_image_shader(t);
 

backends/vulkan/runtime/graph/ops/impl/Sum.cpp

@@ -120,7 +120,7 @@ void add_sum_dim_IntList(
   vTensor& in_tensor = graph.get_val(in).toTensor();
 
   std::set<int64_t> dims_set;
-  auto dims_to_sum = graph.get_val(opt_dim).toIntList();
+  const auto& dims_to_sum = graph.get_val(opt_dim).toIntList();
   int64_t in_dim = in_tensor.sizes().size();
 
   for (const auto& dim : dims_to_sum) {

backends/vulkan/test/vulkan_compute_api_test.cpp

@@ -401,7 +401,7 @@ TEST(VulkanComputeGraphTest, test_values_scalar_list_inplace_constructed) {
   ComputeGraph graph(config);
 
   ValueRef idx = graph.add_scalar_list<int64_t>({1, 2, 3, 4});
-  std::vector<int64_t>& arr = graph.get_val(idx).toIntList();
+  const auto& arr = graph.get_val(idx).toIntList();
   EXPECT_TRUE(arr.size() == 4);
   for (int i = 0; i < 4; i++) {
     EXPECT_TRUE(arr[i] == i + 1);
@@ -417,7 +417,7 @@ TEST(VulkanComputeGraphTest, test_values_scalar_list_outside_constructed) {
     std::vector<double> data = {5.0, 4.0, 3.0, 2.0, 1.0};
     idx = graph.add_scalar_list(std::move(data));
   }
-  std::vector<double>& arr = graph.get_val(idx).toDoubleList();
+  const auto& arr = graph.get_val(idx).toDoubleList();
   EXPECT_TRUE(arr.size() == 5);
   for (int i = 0; i < 5; i++) {
     EXPECT_TRUE(arr[i] == (5 - i));