add clear_gradients to star gan ut, test=develop (#23296)

python/paddle/fluid/tests/unittests/test_imperative_star_gan_with_gradient_penalty.py

@@ -482,6 +482,13 @@ class DyGraphTrainModel(object):
         self.backward_strategy = fluid.dygraph.BackwardStrategy()
         self.backward_strategy.sort_sum_gradient = cfg.sort_sum_gradient
 
+    def clear_gradients(self):
+        if self.g_optimizer:
+            self.g_optimizer.clear_gradients()
+
+        if self.d_optimizer:
+            self.d_optimizer.clear_gradients()
+
     def run(self, image_real, label_org, label_trg):
         image_real = fluid.dygraph.to_variable(image_real)
         label_org = fluid.dygraph.to_variable(label_org)
@@ -493,7 +500,8 @@ class DyGraphTrainModel(object):
         g_loss.backward(self.backward_strategy)
         if self.g_optimizer:
             self.g_optimizer.minimize(g_loss)
-            self.generator.clear_gradients()
+
+        self.clear_gradients()
 
         d_loss = get_discriminator_loss(image_real, label_org, label_trg,
                                         self.generator, self.discriminator,
@@ -501,7 +509,8 @@ class DyGraphTrainModel(object):
         d_loss.backward(self.backward_strategy)
         if self.d_optimizer:
             self.d_optimizer.minimize(d_loss)
-            self.discriminator.clear_gradients()
+
+        self.clear_gradients()
 
         return g_loss.numpy()[0], d_loss.numpy()[0]