[DataLoader] Add Example Only `fork` DataPipe (#60679)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60679
This is example only DataPipe, not intended to be used in production. Used for tutorials, tests and documentation.
Have to be replaced by real `fork` upon DataLoader update.
Test Plan: Imported from OSS
Reviewed By: mruberry
Differential Revision: D29461084
Pulled By: VitalyFedyunin
fbshipit-source-id: a7e435f055f040e358f5465092b8daa07f8e29b7
