Refractor flatbuffer jit code
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75239
Refractor flatbuffer_serializer to move JIT related code to a separate file .
Differential Revision: [D35301020](https://our.internmc.facebook.com/intern/diff/D35301020/)
**NOTE FOR REVIEWERS**: This PR has internal Facebook specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D35301020/)!
Approved by: https://github.com/iseeyuan
