Create slot variables for Adam with accumulated gradients.

def _create_slots(self, var_list):
    """Create slot variables for Adam with accumulated gradients."""
    super(MultistepAdamOptimizer, self)._create_slots(var_list)
    first_var = min(var_list, key=lambda x: x.name)
    self._create_non_slot_variable(initial_value=0 if self._n == 1 else 1,
                                   name="iter",
                                   colocate_with=first_var)
    for v in var_list:
      self._zeros_slot(v, "grad_acc", self._name)