New flatbuffer_loader functions that do not depend on flatbuffers.h (#82618)
This is the first step towards hiding the flatbuffers types and headers from the load/serialize APIs. The two new functions make it possible to load modules without using `GetMutableModule` (defined by the generated header) or `FlatbufferLoader` (which depends on flatbuffers types).
D38292794 will remove the functions/class marked DEPRECATED here after migrating existing users.
Differential Revision: [D38292793](https://our.internmc.facebook.com/intern/diff/D38292793/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82618
Approved by: https://github.com/qihqi
