[PyTorch] add intrusive_ptr exclusive ownership benchmark
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76766
None of the existing benchmarks captured the cost of the final destruction of an intrusive_ptr.
Differential Revision: [D36109756](https://our.internmc.facebook.com/intern/diff/D36109756/)
Approved by: https://github.com/ezyang
