Replace group_by_key by group_by IterDataPipe (#64220)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64220
Remove `ByKeyGrouperIterDataPipe` due to duplicated functionality.
Fix a bug in `GrouperIterDataPipe` using the existing test.
Test Plan: Imported from OSS
Reviewed By: VitalyFedyunin
Differential Revision: D30650542
Pulled By: ejguan
fbshipit-source-id: 666b4d28282fb4f49f3ff101b8d08be16a50d836
