move distributed_init after get_batch_iterator

Summary: There are constantly wait timeout issue for using multiple nodes, even setting copylocallytempdir:/ doesn't help, eg f105637629. It seems to be working after I moved distributed_init after get_batch_iterator, eg f106520580

Reviewed By: myleott

Differential Revision: D14817769

fbshipit-source-id: edbb101a28d8082241c7bdd8c5500c9dad27647c