[Model Averaging] Add a unit test that launches hierarchical SGD by PostLocalSGDOptimizer (#74668)
Summary:
As title.
The added unit test requires 4 GPUs. Please add `ciflow/all` to enable this test.
Proposal: https://github.com/pytorch/pytorch/issues/73382
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74668
Reviewed By: albanD
Differential Revision: D35173938
Pulled By: mrshenli
fbshipit-source-id: b6d61822bfa12c793050af96a8baa4fc92f6b120
(cherry picked from commit 998a2d28ff61a413a1ad516e2c45dcc00a180499)
