[ET-VK] Using vector for storing ref_mapping_ in GraphBuilder to improve model load time and memory. (#10689)

Pull Request resolved: #10647

This diff changes GraphBuilder class to store ref id to value mapping as vector instead of unordered map, since maximum id is known and thus vector can be sized to store the map.
ghstack-source-id: 282013578
@exported-using-ghexport

Differential Revision: [D73969916](https://our.internmc.facebook.com/intern/diff/D73969916/)