fix annotation for Demultiplexer (#65998)
Summary:
cc SsnL VitalyFedyunin ejguan NivekT
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65998
Reviewed By: bdhirsh
Differential Revision: D32145926
Pulled By: ejguan
fbshipit-source-id: 60be3126fb9e73b8631b5040676264504e926707
