Remove flatbuffer types/headers from flatbuffer_serializer[_jit].h (#82619)
Hide the flatbuffers types and headers from the serialize APIs, and stop using the DEPRECATED functions from flatbuffer_loader.h.
This required creating the new `DetachedBuffer` type to replace/hide `flatbuffers::DetachedBuffer`, a class that owns a span of custom-allocated memory.
This is another step towards hiding the flatbuffers types and headers from the load/serialize APIs.
Differential Revision: [D38292798](https://our.internmc.facebook.com/intern/diff/D38292798/)
**NOTE FOR REVIEWERS**: This PR has internal Facebook specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D38292798/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82619
Approved by: https://github.com/qihqi
