Make Demux serializable with lambda function (#71311)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/71311
Test Plan: Imported from OSS
Reviewed By: NivekT
Differential Revision: D33584552
Pulled By: ejguan
fbshipit-source-id: 52324faf5547f9f77582ec170ec91ce3114cfc61
