tear down torch dist pg when test completes

tests/unit/common.py

@@ -51,6 +51,8 @@ def dist_init(local_rank, num_procs, *func_args, **func_kwargs):
 
             # make sure all ranks finish at the same time
             torch.distributed.barrier()
+            # tear down after test completes
+            torch.distributed.destroy_process_group()
 
         def dist_launcher(num_procs, *func_args, **func_kwargs):
             """Launch processes and gracefully handle failures. """