message: Fixes Github
  Fixes b/164972447
  Raises TypeError if None object is passed as target to gradient.
diff: @@ -1007,6 +1007,7 @@ class GradientTape(object):
  Raises:
  RuntimeError: If called on a used, non-persistent tape.
  RuntimeError: If called inside the context of the tape.
  + TypeError: If the target is a None object.
  ValueError: If the target is a variable or if unconnected gradients is
  called with an unknown value.
  """
  @@ -1028,6 +1029,11 @@ class GradientTape(object):
  "gradient in order to compute higher order "
  "derivatives.", 1)
  + if target is None:
  + raise TypeError("Target should be a list or nested structure"
  + " of Tensors or Variables to be differentiated,"
  + " but recieved %r" % (target))
  +
  num_ndarrays = 0
  flat_targets = []
  for t in nest.flatten(target):
  