Additional autograd unit tests for Python UDFs. (#29041)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29041
1) Enhanced autograd unit tests to test the
torch.distributed.autograd.backward() API more thoroughly on Python UDFs.
2) Enhanced `python_error` to override `what` such that it returns an
appropriate error string if we call `what()` on this error. This ensures we can
propagate exceptions over the wire during RPCs (since we get the error string
by calling what() on the exception)
ghstack-source-id: 93098679
ghstack-source-id: 93098679
Test Plan: waitforbuildbot
Reviewed By: mrshenli
Differential Revision: D18273041
fbshipit-source-id: 85d3932fed6337668a812367fdfce233c1b3ff8e
