Tells plasma to prefetch the given object_id.

def plasma_prefetch(object_id):
    """Tells plasma to prefetch the given object_id."""
    local_sched_client = ray.worker.global_worker.raylet_client
    ray_obj_id = ray.ObjectID(object_id)
    local_sched_client.fetch_or_reconstruct([ray_obj_id], True)