[DataLoader] Typing Doc (#54773)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/54773
Test Plan: Imported from OSS
Reviewed By: VitalyFedyunin
Differential Revision: D27364246
Pulled By: ejguan
fbshipit-source-id: 48908555853c364d2d3cc173e3b73a6bec2e19f1
