Provide async mode for local autograd engine. (#31230)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31230
A major issue with distributed autograd currently is that we block an
RPC thread when we call Engine::execute_with_graph_task.
To resolve this issue, I've made modifications to the local autograd engine
such that `execute_with_graph_task` returns a Future instead. The `execute()`
methods for Engine::execute() and DistEngine::execute() still wait() on this
Future which ensures there is no change in behavior yet.
In follow up PRs we can modify the distributed autograd engine to take
advantage of this Future.
Closes #26359
ghstack-source-id: 96298057
Test Plan: waitforbuildbot
Differential Revision: D18999709
fbshipit-source-id: 388f54467fd2415a0acb7df17bd063aedc105229
