Release GIL when deleting users and unforked owners (#39555)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39555
This function does not require GIL, as all OwnerRRef-related
py::object deletion is now guarded by ConcretePyObjectHolder. If
we hold lock here, we could potentially run into deadlock, if there
are other threads in the RPC thread pool trying to acquire GIL to
destruct Python UDFs or OwnerRRefs.
Test Plan: Imported from OSS
Differential Revision: D21897125
Pulled By: mrshenli
fbshipit-source-id: 96157689df38bc409af57b83248ae73823d1f959
