[PyTorch] intrusive_ptr: don't guarantee release_resources will be called
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76767
We're spending a virtual function call in the common case
where there are no weak references just to save a small amount of care
in intrusive_ptr_target subclasses that override release_resources, of
which there aren't very many.
Differential Revision: [D36109757](https://our.internmc.facebook.com/intern/diff/D36109757/)
**NOTE FOR REVIEWERS**: This PR has internal Facebook specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D36109757/)!
Approved by: https://github.com/ezyang
