Average aggregated gradients before put in plasma store (#3631)

python/ray/experimental/sgd/param_server.py

@@ -67,6 +67,7 @@ class ParameterServer(object):
         client = ray.worker.global_worker.plasma_client
         assert self.acc_counter == self.num_sgd_workers, self.acc_counter
         oid = ray.pyarrow.plasma.ObjectID(object_id)
+        self.accumulated /= self.acc_counter
         client.put(self.accumulated.flatten(), object_id=oid)
         self.accumulated = np.zeros_like(self.accumulated)
         self.acc_counter = 0