Overriding gradient for shake-shake of 2 tensors.

def shakeshake2_indiv_grad(x1, x2, dy):
  """Overriding gradient for shake-shake of 2 tensors."""
  y = shakeshake2_py(x1, x2, individual=True)
  dx = tf.gradients(ys=[y], xs=[x1, x2], grad_ys=[dy])
  return dx