message: Removes use of _grad_fn_accepts_none_for_indices in magic_gradient_function.
  Leaves the one in imperative_grad, which seems to matter.
diff: @@ -388,12 +388,6 @@ def _magic_gradient_function(op_name, attr_tuple, num_inputs,
  if grad_fn is None:
  return [None] * num_inputs
  - none_indices = _grad_fn_accepts_none_for_indices.get(op_name, [])
  - out_grads = [
  - o if (o is not None or i in none_indices)
  - else array_ops.zeros_like(outputs[i])
  - for i, o in enumerate(out_grads)
  - ]
  return grad_fn(mock_op, *out_grads)
  