[DataPipe] improving DataPipe unit tests (#70215)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70215
A few renaming, formatting, and additional tests to make the unit tests better.
cc VitalyFedyunin ejguan NivekT
Test Plan: Imported from OSS
Reviewed By: ejguan
Differential Revision: D33344610
Pulled By: NivekT
fbshipit-source-id: bb36f7452bdc44964c9ce0650c7ae308ba2c5aa5
(cherry picked from commit 0aae20cb27038b7b3598520db4304a604f1e6799)
