[ET-VK] lowering ExecuTorch tensor dtype for Vulkan tensor dtype to enable 64bit

backends/vulkan/runtime/VulkanBackend.cpp

@@ -599,12 +599,45 @@ class VulkanBackend final : public ::executorch::runtime::BackendInterface {
       if (compute_graph->val_is_tensor(oref)) {
         VK_CHECK_COND(args[o]->isTensor());
         maybe_resize_output(compute_graph, i, args[o]->toTensor());
+
+        // Get the Vulkan tensor dtype and ExecutorTorch tensor dtype
+        vTensorPtr vulkan_tensor = compute_graph->get_tensor(oref);
+        vkapi::ScalarType vulkan_dtype = vulkan_tensor->dtype();
+        executorch::aten::ScalarType et_dtype =
+            args[o]->toTensor().scalar_type();
+
         // args holds inputs directly followed by outputs, so the i'th output
         // for compute_graph corresponds to the o'th arg
         compute_graph->copy_from_staging(
             compute_graph->outputs()[i].staging,
             args[o]->toTensor().mutable_data_ptr(),
             args[o]->toTensor().numel());
+
+        // Handle dtype conversion between Vulkan and ExecutorTorch (in-place)
+        if (vulkan_dtype == vkapi::kFloat &&
+            et_dtype == executorch::aten::ScalarType::Double) {
+          // Convert float32 to float64 in-place (backwards to avoid
+          // overwriting)
+          double* data_64 = args[o]->toTensor().mutable_data_ptr<double>();
+          const float* data_32 = args[o]->toTensor().const_data_ptr<float>();
+          for (size_t j = args[o]->toTensor().numel() - 1; j >= 0; --j) {
+            data_64[j] = static_cast<double>(data_32[j]);
+            if (j == 0)
+              break; // Prevent underflow for size_t
+          }
+        } else if (
+            vulkan_dtype == vkapi::kInt &&
+            et_dtype == executorch::aten::ScalarType::Long) {
+          // Convert int32 to int64 in-place (backwards to avoid overwriting)
+          int64_t* data_64 = args[o]->toTensor().mutable_data_ptr<int64_t>();
+          const int32_t* data_32 =
+              args[o]->toTensor().const_data_ptr<int32_t>();
+          for (size_t j = args[o]->toTensor().numel() - 1; j >= 0; --j) {
+            data_64[j] = static_cast<int64_t>(data_32[j]);
+            if (j == 0)
+              break; // Prevent underflow for size_t
+          }
+        }
       }
       // TensorRef values represent constant tensors which will not have been
       // modified by the graph execution. Therefore, if a constant tensor is