[DataPipe] adding description, __len__, tests for mux() (#64224)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64224
cc VitalyFedyunin ejguan
Test Plan: Imported from OSS
Reviewed By: ejguan
Differential Revision: D30651551
Pulled By: NivekT
fbshipit-source-id: f8af98ba71a592900b992a8077432062ec57bb48
