modify code style

fluid/adversarial/advbox/attacks/iterator_gradientsign.py

@@ -12,7 +12,6 @@ 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.