soft_label_distiller fix, test=release/1.6 (#20657)

Author: baiyfbupt

python/paddle/fluid/contrib/slim/distillation/distiller.py

@@ -264,11 +264,14 @@ def apply(self, graph):
 
             student_feature_map = ret_graph.var(self.student_feature_map)._var
             teacher_feature_map = ret_graph.var(self.teacher_feature_map)._var
-            s_fea = student_feature_map / self.student_temperature
-            t_fea = teacher_feature_map / self.teacher_temperature
+            s_fea = layers.softmax(student_feature_map /
+                                   self.student_temperature)
+            t_fea = layers.softmax(teacher_feature_map /
+                                   self.teacher_temperature)
             t_fea.stop_gradient = True
-            ce_loss = layers.softmax_with_cross_entropy(
-                s_fea, t_fea, soft_label=True)
+            ce_loss = layres.reduce_mean(
+                layers.cross_entropy(
+                    s_fea, t_fea, soft_label=True))
             distillation_loss = ce_loss * self.distillation_loss_weight
             student_loss = 0
             if 'loss' in ret_graph.out_nodes: