remove _register_* bindings from python (#29499)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29499
This changes how DataParallel and trace module creation works so that
we no longer need to mutate Module class after it has been created.
The only remaining usage of register_* functions are now inside C++
tests.
Test Plan: Imported from OSS
Differential Revision: D18413652
Pulled By: zdevito
fbshipit-source-id: f039e5400cd016632768be4547892f6a69645c20
