[DataPipe] implementing __len__ for fork (no valid length for demux) (#64215)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/64215
Test Plan: Imported from OSS
Reviewed By: ejguan
Differential Revision: D30648672
Pulled By: NivekT
fbshipit-source-id: 4780f2f6a79ae15a4009092475e7d92f96dd09a2
