[autograd] Avoid holding lock when completing GraphTask futureResult (#35101)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35101
TSAN is noting lock-order-inversion in context of dist autograd because
we're holding lock when GraphTask calls markCompleted() on the relevant futureResult_.
Add an atomic bool to make it possible to protect this without holding the mutex,
and also fix alignment of a few struct vars.
ghstack-source-id: 101805283
Test Plan: buck test mode/opt-tsan //caffe2/test/distributed/rpc:dist_autograd_spawn_thrift
Differential Revision: D20553517
fbshipit-source-id: 446e3718dd68876bd312166ecceed1d92868ce4e
