Fix memory leak due to circular dependency. (#31030)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31030
DistAutogradContext held a shared_ptr reference to RecvRpcBackward and
RecvRpcBackward held a shared_ptr reference to the context. This circular
dependency caused significant memory leaks. As a result, I'm changing the
reference in RecvRpcBackward to be a weak_ptr.
Test Plan: waitforbuildbot
Differential Revision: D18896389
fbshipit-source-id: e5bc588b6f998885854e3a67de1e82452e8475ce
