Modify inline doc for DataPipe (#64221)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64221
List of tasks in this PR
- [x] Add inline doc for DataPipe
- [x] Improve the inline doc
- [x] Expose DataPipe to `datapipes.iter` (`UnBatcher`) Note: `Forker`, `Demux`, `Mux` are exposed in another PR authored by Kevin
- [x] Add correct typing to DataPipe
- [x] Unify the argument to `datapipe` rather than `source_datapipe`
Test Plan: Imported from OSS
Reviewed By: VitalyFedyunin
Differential Revision: D30650541
Pulled By: ejguan
fbshipit-source-id: c09d1b9742b8097d8e645c15947cef80c876877b
