[PyTorch] Fix write-after-free (TSAN) in GraphTask::set_error() (#33156)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33156
When dist_autograd_spawn_thrift's 'test_backward_node_failure_python_udf' test is
run, it was encountering a TSAN error related to holding the mutex while the
underlying datastructure was being dealloced.
In this change, we simply get a shared_ptr<> reference to the future, and
set_exception() without having the lock held, to avoid deallocing underneath
the lock.
ghstack-source-id: 98303434
Test Plan: buck test mode/opt-tsan //caffe2/test/distributed/rpc:dist_autograd_spawn_thrift -- 'test_backward_node_failure_python_udf \(test_dist_autograd_spawn\.DistAutogradTestWithSpawn\)'
Differential Revision: D19821362
fbshipit-source-id: 82f735e33f8e608552418ae71592400fa3621e40
