[DataLoader] Rename Functional DataSet to DataPipe (#51488)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/51488
Test Plan: Imported from OSS
Reviewed By: H-Huang
Differential Revision: D26209888
Pulled By: ejguan
fbshipit-source-id: cb8bc852b1e4d72be81e0297308a43954cd95332
