Create flatbuffers_mobile (#82828)
Differential Revision: [D38412636](https://our.internmc.facebook.com/intern/diff/D38412636/)
**NOTE FOR REVIEWERS**: This PR has internal Facebook specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D38412636/)!
Differential Revision: [D38412636](https://our.internmc.facebook.com/intern/diff/D38412636)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82828
Approved by: https://github.com/qihqi
