fix globalbatchnorm bug

mindspore/nn/layer/normalization.py

@@ -74,7 +74,7 @@ class _BatchNorm(Cell):
                     management.create_group('group' + str(i), self.rank_list[i])
                     self.all_reduce = P.AllReduce(P.ReduceOp.SUM, 'group' + str(i)).add_prim_attr('fusion', 1)
         self.shape = P.Shape()
-        self.reduce_mean = P.ReduceMean()
+        self.reduce_mean = P.ReduceMean(keep_dims=True)
         self.square = P.Square()
         self.sqrt = P.Sqrt()
         self.cast = P.Cast()