[DataPipe] moving mux IterDataPipe test to the right location (#66277)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66277
Previously, it is grouped together with tests related to `MapDataPipe`, but it should be with `IterDataPipe`.
cc VitalyFedyunin ejguan NivekT
Test Plan: Imported from OSS
Reviewed By: ejguan
Differential Revision: D31485823
Pulled By: NivekT
fbshipit-source-id: d13d8c28cbfc305da0e3033d4109a0f971281a02
