<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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)