[DataPipe] export fork, mux, demux for public usage (#64279)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64279
cc VitalyFedyunin ejguan
Test Plan: Imported from OSS
Reviewed By: ejguan
Differential Revision: D30671971
Pulled By: NivekT
fbshipit-source-id: 056ac12ef7183b254d1eec341145594639e47ef6
