Migrate remaining pytorch code to use new flatbuffer_loader.h APIs (#82620)
This is the only file in pytorch core that refers to the deprecated flatbuffer_loader.h APIs. Move to the non-deprecated functions.
Differential Revision: [D38330369](https://our.internmc.facebook.com/intern/diff/D38330369/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82620
Approved by: https://github.com/qihqi
