Additional autograd unit tests for Python UDFs. (#28824)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28824
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: 92972494
Test Plan: waitforbuildbot
Differential Revision: D18195584
fbshipit-source-id: b795daf644ba1816fdec484545192ab55a2f71e7
