[DataPipe] Fix fork/unzip with a single child (#81502)
When `Forker` or `Unzipper` only contains a single child, the buffer should be cleaned up. This is one of the root causes for the issue reported internally. See: https://fburl.com/2k0et1gv
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81502
Approved by: https://github.com/VitalyFedyunin, https://github.com/NivekT
