Remove flatbuffer types/headers from flatbuffer_loader.h (#82893)
This completely hides the flatbuffer types and headers from users of flatbuffer_loader/serializer, turning them into an internal implementation detail.
A followup diff will fix up the buck files to hide the dependencies more thoroughly.
While doing this I found another use of a flatbuffer-defined name (`FLATBUFFERS_MAX_ALIGNMENT`), which highlighted the issues described in T128189662.
Differential Revision: [D38292794](https://our.internmc.facebook.com/intern/diff/D38292794/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82893
Approved by: https://github.com/qihqi
