Remove dead code in ddp.{h, cpp} (#37990)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37990
The code in `ddp.{h, cpp}` and the corresponding pybind implementations are no longer used. The pybinded calls were all private APIs and only ran in unittests, so we should remove these unused APIs.
https://github.com/pytorch/pytorch/pull/20234 from a year ago also mentioned that we should delete `_dist_broadcast_coalesced`
Verified that all tests pass with cuda by running `test_c10d` on a gpu-enabled machine.
ghstack-source-id: 103885383
Test Plan: CI
Differential Revision: D21443879
fbshipit-source-id: 764d8681ca629056bfe2c260ffab47fa5bdf07ff
