Update on "[ET-VK] Only save_cache the first time"

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.

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

[ghstack-poisoned]

Author: jorgep31415