AsyncIf op implementation
Summary:
This diff adds the following:
- An AsyncIf to support conditional async execution. This op assumes that then_net and else_net are async scheduling nets. This op itself completes when every async op in the active net completes. Cancellation cancels the inner nets and the async ops.
- Unit tests targeting asynchronicity and error/cancellation handling.
Test Plan:
New unit tests
With --stress-runs=2000:
https://our.intern.facebook.com/intern/testinfra/testrun/4785074616784325
Reviewed By: ilia-cher
Differential Revision: D18051357
fbshipit-source-id: 1399a437b3ca63fd4ea0cf08d173f85b9242cc1f
