Fix compare_exchange_weak usage in weak_intrusive_ptr (#16302)
Summary:
In the case of spurious failure, refcount is not incremented -- which leads to underflow once all references are released.
This was discovered when exercising multiprocessing on ppc64le.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16302
Differential Revision: D13845435
Pulled By: ezyang
fbshipit-source-id: 8e264fff9dca8152cb12617e3216d5e48acd9557
