[Model Averaging] Add a reference to hierarchical SGD (#73823)
Summary:
Add a reference.
Also fix the comment: unlike `averagers.py`, currently this is not a base class that can inherit many subclasses.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73823
Reviewed By: ejguan
Differential Revision: D34684366
Pulled By: rohan-varma
fbshipit-source-id: e253ed39ba0783ad73bfd889e9a2e7d0c9214a3a
(cherry picked from commit a9fec3585078881ccd5886ebb27e52b15f7181b1)
