[DataPipe] apply dill serialization for _Demux and add cache to traverse
- Fix _Demux can not be pickled with DILL presented https://github.com/pytorch/pytorch/pull/74958#issuecomment-1084637227
- And add cache to traverse function to prevent infinite recursion for circular reference of DataPipe (Fixes https://github.com/pytorch/data/issues/237)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75034
Approved by: https://github.com/wenleix
