Remove useless _allgather_then_aggregate_hook (#52593)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52593
This hook is not used at all, and it probably can only be used for demonstrating that allgather is slower than allreduce, so it should never be used in practice.
However, this hook and its helper function stay with the communication hook public APIs in the same file. It will be better to make the public API file as concise as possible.
Since I don't think we will use this hook in the future, prefer deleting it to moving it to a separate file.
ghstack-source-id: 122180969
Test Plan: waitforbuildbot
Reviewed By: rohan-varma
Differential Revision: D26575318
fbshipit-source-id: b258154a7c92e33236c34104bd79bc244ecdb158
