All variables used by a function wrapped with @custom_gradient must be `ResourceVariable`s. Ensure that no `variable_scope` is created with `use_resource=False`.

Raise code

v.ref() for v in current_var_scope.global_variables() +
      current_var_scope.local_variables()
  ])
  new_vars = after_vars - before_vars
  new_vars_list = [v.deref() for v in new_vars]
  for v in new_vars_list:
    if not resource_variable_ops.is_resource_variable(v):
      raise TypeError(
          "All variables used by a function wrapped with @custom_gradient must "
          "be `ResourceVariable`s. Ensure that no `variable_scope` is created "
          "with `use_resource=False`.")

  # The variables that grad_fn needs to return gradients for are the set of
  # variables used that are *not* part of the inputs.
  variables_in_tape = frozenset([