[DataPipe] fixing tests related fork() to remove warnings (#64827)
Summary:
There are two warnings produced by `test_fork_datapipe`. This PR addresses the issues raised by those warnings without impacting the test cases.
cc VitalyFedyunin ejguan
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64827
Reviewed By: ejguan
Differential Revision: D30870528
Pulled By: NivekT
fbshipit-source-id: 580a001c6fa3ff6f8b04a7e5183e58861938204b
