[aten] Don't deadlock in IValue::Future impl, tests. (#34099)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34099
This change effectively applies into IValue's future impl a few fixes
we discovered when using the torch::utils::Future<T> impl.
The parallel impls should probably eventually be merged, but until then:
- Don't hold the lock when invoking the callbacks. This makes
it effectively impossible (deadlocks) to call value() to get
the value from inside the callback.
- We discovered that it was slightly cleaner in practice to
notify condition variables prior to invoking callbacks
(best to unblock paused threads ASAP, before spawning new work).
- Fix some var naming inconsistency.
- Add a some caffe2 cpp test coverage.
ghstack-source-id: 99336569
Test Plan:
```
buck test mode/dev //caffe2/test/cpp/jit:jit -- 'JitTest\.IValueFuture'
```
Differential Revision: D20203278
fbshipit-source-id: 6e805ba547899dab9aab458e4b23049db31f930e
