[rref] Handle exceptions returned via remote() calls (#35331)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35331
When the function called by remote() throws, it seems sensible to
surface that exeption when rref.to_here() is called.
Doing this only involves simple modifications:
- we need the OwnerRRef to keep around an optional<string>
for the error
- add an OwnerRRef setError() method that's parallel to setValue(),
and plumb through the logic
We add rpc_tests to verify that the exception is propagated properly.
ghstack-source-id: 101136900
Test Plan:
buck test mode/dev-nosan caffe2/test/distributed/rpc:rpc_spawn
buck test mode/dev-nosan caffe2/test/distributed/rpc/jit:rpc_spawn
Differential Revision: D20634078
fbshipit-source-id: b5b13fdb85cdf6a43f42347d82eabae1635368ec
