A tensorflow variable tranfomed to be constrained in a L2 unit ball.

  EXPERIMENTAL: Do not use for adverserial examples if you need to be confident
  they are strong attacks. We are not yet confident in this code.

def unit_ball_L2(shape):
  """A tensorflow variable tranfomed to be constrained in a L2 unit ball.

  EXPERIMENTAL: Do not use for adverserial examples if you need to be confident
  they are strong attacks. We are not yet confident in this code.
  """
  x = tf.Variable(tf.zeros(shape))
  return constrain_L2(x)