[Model Averaging] Make HierarchicalModelAverager a subclass of averagers.ModelAverager
Make `HierarchicalModelAverager` a subclass of `averagers.ModelAverager` is a preparation step for incorporating hierarchical SGD into `PostLocalSGDOptimizer`.
Proposal: https://github.com/pytorch/pytorch/issues/73382
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74564
Approved by: https://github.com/rohan-varma
