Refactor flatbuffer loader to allow overriding how IValues are parsed. (#71661)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71661
https://docs.google.com/document/d/1OoPKREoqNbOUIcbGzfk8TTIibeTgx3c6Lr3NthF7-PM/edit
Test Plan: unittest
Reviewed By: zhxchen17
Differential Revision: D33720630
fbshipit-source-id: da24993cf5568c689cb6fda64ba4943d77f8b5e6
(cherry picked from commit 327cf75d234ee2b1aea79dc909b890b96927f536)
