code: def distance(self) -> Distance:
      if self._distance is None:
          # we delay the error until the distance is needed,
          # e.g. when __call__ is executed (that way, run
          # can be used without specifying a distance)
          raise ValueError(
              "unknown distance, please pass `distance` to the attack initializer"
          )
      return self._distance
  
imports: from typing import Callable, TypeVar, Any, Union, Optional, Sequence, List, Tuple, Dict
  from typing_extensions import final, overload
  from abc import ABC, abstractmethod
  from collections.abc import Iterable
  import eagerpy
  from ..models import Model
  from ..criteria import Criterion
  from ..criteria import Misclassification
  from ..devutils import atleast_kd
  from ..distances import Distance
label_desc: Use image node_id 2 for calling the FlexibleDistanceMinimizationAttack obj's underlying member method code with example usage: obj.distance() and returns: self