[ET-VK] Only save_cache the first time

Pull Request resolved: #6966

Add a check in `save_cache` to return early if the cache file already exists. Currently we append the same cache data to that file which makes no difference to model-load time.
ghstack-source-id: 254701486
@exported-using-ghexport

Differential Revision: [D66179919](https://our.internmc.facebook.com/intern/diff/D66179919/)

Author: jorgep31415

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);