[engine] Ensure future is complete when exiting Engine::mark_graph_task_completed() (#36856)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36856
Previously, we could early-exit mark_graph_task_completed() without the future
actually being fully complete - we were only guaranteeing that it was at least
in the process of being marked complete.
This seems to be triggering an assert graph_task->future_result_->completed()
This change simply adds a 1-line waitNoThrow() call to ensure that the future
has been marked complete before exiting the mark_graph_task_completed() function.
The cost is relatively reasonable, since this isn't the common path.
ghstack-source-id: 102423589
Test Plan: buck test mode/dev-nosan caffe2/test/,,,
Differential Revision: D21104121
fbshipit-source-id: 51c1554618880fe80d52d5eb96716abc15f6be8a
