Release DistAutogradContainer context for each dist_autograd test case (#38711)
Summary:
this fixes - https://github.com/pytorch/pytorch/issues/38710
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38711
Differential Revision: D22132057
fbshipit-source-id: 894280d164543c63beaec679c18f2059e7055b01
