message: Update backprop.py
  fixed this bug:
  the gradient function returned by tfe.implicit_value_and_gradients() doesn't support keyword argument
diff: @@ -196,11 +196,11 @@ def implicit_val_and_grad(f):
  # TODO(cais): Remove calls to tf.constant() once the gradients functions
  # accept lists and np.ndarrays.
  - def grad_fn(*args):
  + def grad_fn(*args, **kwds):
  """Computes the gradient of the wrapped function."""
  this_tape = tape.push_new_tape()
  try:
  - end_node = f(*args)
  + end_node = f(*args, **kwds)
  if end_node is None:
  raise ValueError("Cannot differentiate a function that returns None; "
  "did you forget to return a value from {}?".format(
  