Generate adversarial examples and return them as a NumPy array.

    :param x_val: A NumPy array with the original inputs.
    :param **kwargs: optional parameters used by child classes.
    :return: A NumPy array holding the adversarial examples.

def generate_np(self, x_val, **kwargs):
    """
    Generate adversarial examples and return them as a NumPy array.

    :param x_val: A NumPy array with the original inputs.
    :param **kwargs: optional parameters used by child classes.
    :return: A NumPy array holding the adversarial examples.
    """
    tfe = tf.contrib.eager
    x = tfe.Variable(x_val)
    adv_x = self.generate(x, **kwargs)
    return adv_x.numpy()