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
Test Plan: Two tests were added to cuda_stream_test for this functionality.
Differential Revision: D17145538
Pulled By: mruberry
fbshipit-source-id: 2546c5907c038412e03aa0d3328a972b0164c455
