message: TFE: Simplify code example in backprop.py
diff: @@ -686,8 +686,8 @@ def gradients_function(f, params=None):
  # `gradients_function()`.
  ygrad_fn = tfe.gradients_function(f, params=[1])
  - grads = ygrad_fn(x, y)
  - assert grads[0].numpy() == (2 ** 3) - 2 * 2 * 3
  + (y_grad,) = ygrad_fn(x, y)
  + assert y_grad.numpy() == (2 ** 3) - 2 * 2 * 3
  ```
  Args:
  @@ -745,9 +745,9 @@ def val_and_grad_function(f, params=None):
  # argument with `value_and_gradients_function()`.
  val_ygrad_fn = tfe.value_and_gradients_function(f, params=[1])
  - f_val, grads = val_ygrad_fn(x, y)
  + f_val, (y_grad,) = val_ygrad_fn(x, y)
  assert f_val.numpy() == (2 ** 3) * 3 - 2 * (3 ** 2)
  - assert grads[0].numpy() == (2 ** 3) - 2 * 2 * 3
  + assert y_grad.numpy() == (2 ** 3) - 2 * 2 * 3
  ```
  Args:
  