Tighten up invariants regarding StreamId. (#15125)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15125
I realized that it is really bad juju if you fake a StreamId
out of thin air, because in general this isn't going to work.
So, make the constructor a lot scarier.
Most "faking StreamId out of thin air" happens because someone
just wants to put something on the default stream.
Reviewed By: dzhulgakov
Differential Revision: D13432800
fbshipit-source-id: a86991d6fc1d8aa4e54e8175e5f06f90856238e6
