class_docstr: Hypervolume calculator for any dimension.
  
  This class exactly calculates the hypervolume for any dimension by using the WFG algorithm.
  For detail, see `While, Lyndon, Lucas Bradstreet, and Luigi Barone. "A fast way of
  calculating exact hypervolumes." Evolutionary Computation, IEEE Transactions on 16.1 (2012)
  : 86-95.`.
code: def __init__(self) -> None:
      self._reference_point: Optional[np.ndarray] = None
  
file_path: optuna/optuna/_hypervolume/wfg.py
imports: from typing import Optional
  import numpy
  from optuna._hypervolume import _compute_2d
  from optuna._hypervolume import _compute_2points_volume
  from optuna._hypervolume import BaseHypervolume
label_desc: Use image node_id 1 to create a new WFG object from inherited base classes: BaseHypervolume with example: obj = WFG()