message: Fix grad check compatibility
  Relevant [SO](https://stackoverflow.com/questions/58261348/valueerror-tried-to-convert-y-to-a-tensor-and-failed-error-none-values-not)
diff: @@ -89,7 +89,7 @@ class Optimizer(object):
  function not implemented).
  """
  grads = K.gradients(loss, params)
  - if None in grads:
  + if any([g is None for g in grads]):
  raise ValueError('An operation has `None` for gradient. '
  'Please make sure that all of your ops have a '
  'gradient defined (i.e. are differentiable). '
  