[Pytorch] Propagate errors in clearAndWaitForOutstandingRpcsAsync. (#32952)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32952
When the Async() version of clearAndWaitForOutstandingRpcs() was written,
we didn't yet have the generic Future<T> class, and hadn't worked out our
error model fully.
This change fixes that method to properly propagate the first encountered error
to the future, using a bool+CAS.
ghstack-source-id: 97665749
Test Plan: existing test coverage, buck test mode/dev-nosan caffe2/test/...
Differential Revision: D19710337
fbshipit-source-id: 66ce5593a94a16ea624930dbb9409917ef5cfd5d
