Fix load_extra_only api for flatbuffers and enable flatbuffers in mobile for OSS properly (#83855)
`_load_extra_only_for_mobile` API hasn't handled flatbuffers logic yet. Update the api accordingly.
Also find out mobile build in OSS doesn't build with flatbuffers. Filed task T129996445 to track
Differential Revision: [D38890847](https://our.internmc.facebook.com/intern/diff/D38890847/)
**NOTE FOR REVIEWERS**: This PR has internal Facebook specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D38890847/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83855
Approved by: https://github.com/qihqi
