[ET-VK] Only save_cache the first time

backends/vulkan/runtime/vk_api/Pipeline.cpp

@@ -433,12 +433,12 @@ void ComputePipelineCache::purge() {
 }
 
 std::vector<char> ComputePipelineCache::load_cache() {
-  // Return if path is not specified; this means the optimization is disabled
+  // No optimization if path is unspecified
   if (cache_data_path_.empty()) {
     return {};
   }
 
-  // Return if file doesn't exist; this is expected on the first model-load
+  // Return if file doesn't exist; this is expected on first model-load
   std::ifstream file(cache_data_path_, std::ios::binary | std::ios::ate);
   if (file.fail()) {
     return {};
@@ -454,6 +454,17 @@ std::vector<char> ComputePipelineCache::load_cache() {
 }
 
 void ComputePipelineCache::save_cache() {
+  // No optimization if path is unspecified
+  if (cache_data_path_.empty()) {
+    return;
+  }
+
+  // Return if file exists; the cache is already saved
+  std::ifstream ifile(cache_data_path_);
+  if (ifile.good()) {
+    return;
+  }
+
   size_t size{};
   vkGetPipelineCacheData(device_, pipeline_cache_, &size, nullptr);