[futures] Add assert to Future constValue() accessor, add hasValue(). (#39950)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39950
Per the comment in the code, constValue() should only be used in
the case where the future was complete and value was not an error.
Add an assert to enforce this.
Also, add hasValue() accessor for completeness.
ghstack-source-id: 105815597
Test Plan: buck test mode/dev-nosan caffe2/test/cpp/jit:
Differential Revision: D22021776
fbshipit-source-id: b59b6c775eab344068a76f4cd8c3a9dc1f2a174e
