message: Fix Optimizer "No gradients provided" error messages to report variables instead of internal processor objects.
diff: @@ -585,7 +585,7 @@ class Optimizer(
  var_list = [v for g, v, _ in converted_grads_and_vars if g is not None]
  if not var_list:
  raise ValueError("No gradients provided for any variable: %s." %
  - ([str(v) for _, _, v in converted_grads_and_vars],))
  + ([str(v) for _, v, _ in converted_grads_and_vars],))
  with ops.init_scope():
  self._create_slots(var_list)
  update_ops = []
  