Hide flatbuffer build dependencies (#82953)
Lock down the visibility and exporting of flatbuffer implementation details. Clients should not be able to see the generated header, and should avoid picking up the flatbuffer deps if possible.
Differential Revision: [D38495231](https://our.internmc.facebook.com/intern/diff/D38495231/)
**NOTE FOR REVIEWERS**: This PR has internal Facebook specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D38495231/)!
Differential Revision: [D38495231](https://our.internmc.facebook.com/intern/diff/D38495231)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82953
Approved by: https://github.com/cccclai
