add barrier before process group cleanup

iiuc not necessary because our ops are all sync but good to include in case users take this code and run more complex scripts with it

14_clusters/simple_torch_cluster_script.py

@@ -87,6 +87,7 @@ def init_processes(backend):
         dist.init_process_group(backend, rank=WORLD_RANK, world_size=WORLD_SIZE)
         yield
     finally:
+        dist.barrier()  # ensure any async work is done before cleaning up
         # Remove this if it causes program to hang. ref: https://github.com/pytorch/pytorch/issues/75097.
         dist.destroy_process_group()