class_docstr: Base class for a forecast-based anomaly detector.
code: def _get_anomaly_score(
      self,
      time_series: pd.DataFrame,
      time_series_prev: pd.DataFrame = None,
  ) -> pd.DataFrame:
      raise NotImplementedError(
          "_get_anomaly_score() should not be called from a forecast-based anomaly detector."
      )
  
imports: import copy
  import logging
  from typing import List, Optional, Tuple, Union
  import numpy
  import pandas
  from merlion.models.anomaly.base import DetectorBase
  from merlion.models.forecast.base import ForecasterBase
  from merlion.plot import Figure
  from merlion.utils import TimeSeries
  from merlion.utils.misc import AutodocABCMeta
label_desc: Use image node_id 6 for calling the ForecastingDetectorBase obj's underlying member method code with example usage: obj._get_anomaly_score(time_series, time_series_prev) without return types