Creates Torch-friendly Event class and adds Stream tracking to autograd (#25130)
Summary:
Resubmission of https://github.com/pytorch/pytorch/issues/23424 because previous PR was borked.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25130
Differential Revision: D17052534
Pulled By: mruberry
fbshipit-source-id: d91b308ad0f730646bb7b3492a601cd9b05c72d8
