Update multiprocessing note now that shared CUDA tensors are refcounted (#19904)
Summary:
The mp notes are not updated after https://github.com/pytorch/pytorch/pull/16854. (The torch.multiprocessing page is.)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19904
Differential Revision: D15509661
Pulled By: soumith
fbshipit-source-id: 7c11e14a6c804498dda3adbf19710e63e6a564a0
