modify code style

fluid/adversarial/advbox/attacks/iterator_gradientsign.py

@@ -12,6 +12,7 @@ class IteratorGradientSignAttack(Attack):
     This attack was originally implemented by Alexey Kurakin(Google Brain).
     Paper link: https://arxiv.org/pdf/1607.02533.pdf
     """
+
     def _apply(self, image_label, epsilons=100, steps=10):
         """
         Apply the iterative gradient sign attack.
@@ -22,7 +23,6 @@ class IteratorGradientSignAttack(Attack):
         Return:
             numpy.ndarray: The adversarail sample generated by the algorithm.
         """
-
         assert len(image_label) == 1
         pre_label = np.argmax(self.model.predict(image_label))
         gradient = self.model.gradient(image_label)
@@ -41,4 +41,3 @@ class IteratorGradientSignAttack(Attack):
                 adv_label = np.argmax(self.model.predict([(adv_img, 0)]))
                 if pre_label != adv_label:
                     return adv_img
-