renaming TestDdpCommHook class so it doesn't get picked up as a test by pytest (#44905)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/44905
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D23825308
Pulled By: bdhirsh
fbshipit-source-id: 17a07b3bd211850d6ecca793fd9ef3f326ca9274
