Update on "[ET-VK] Fix VulkanBackend args output index"

The one in the `VK_CHECK_COND` expression was wrong. Use a variable for easier tracking.

Differential Revision: [D67126361](https://our.internmc.facebook.com/intern/diff/D67126361/)

[ghstack-poisoned]

Author: jorgep31415

backends/vulkan/runtime/VulkanBackend.cpp

@@ -582,7 +582,7 @@ class VulkanBackend final : public ::executorch::runtime::BackendInterface {
     compute_graph->execute();
 
     for (size_t i = 0; i < compute_graph->outputs().size(); i++) {
-      const size_t o = num_inputs + i;
+      const size_t o = i + num_inputs;
       const ValueRef oref = compute_graph->outputs()[i].value;
       if (compute_graph->val_is_tensor(oref)) {
         VK_CHECK_COND(args[o]->isTensor());