!7391 Fix SmoothL1Loss op sigma attr problem.

Merge pull request !7391 from liangchenghui/fix_smooth_l1_loss

mindspore/ops/operations/nn_ops.py

@@ -1884,6 +1884,7 @@ class SmoothL1Loss(PrimitiveWithInfer):
         validator.check_value_type('beta', beta, [float], self.name)
         validator.check('beta', beta, '', 0, Rel.GT, self.name)
         self.init_prim_io_names(inputs=['prediction', 'target'], outputs=['output'])
+        self.add_prim_attr('sigma', beta)
 
     def infer_shape(self, prediction, target):
         validator.check('prediction shape', prediction, 'target shape', target, Rel.EQ, self.name)