Decouple MapDataPipe from Dataset (#70991)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/70991
Test Plan: Imported from OSS
Reviewed By: dagitses
Differential Revision: D33477680
Pulled By: ejguan
fbshipit-source-id: d3e89492e921a96791319f35052a229684ddf7cf
