Revert [DataPipe] Update mux data pipe
Reverts #76384
this this is breaking tests test_demux_mux_datapipe (__main__.TestIterableDataPipeBasic. See logs: https://hud.pytorch.org/pytorch/pytorch/commit/a997046017aa9f97213f6bd50bc152a6445e8086
and was red on the PR as well: https://hud.pytorch.org/pytorch/pytorch/pull/76384
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76507
Approved by: https://github.com/kit1980
