code: def compute_metric(
      self,
      data_future: TimeSeriesDataFrame,
      predictions: TimeSeriesDataFrame,
      target: str = "target",
      **kwargs,
  ) -> float:
      return (
          (data_future[target] - predictions["mean"])
          / self._past_target_mean
      ).mean()
  
file_path: autogluon/timeseries/tests/unittests/common.py
imports: import random
  from typing import Dict, List, Optional, Union
  import numpy
  import pandas
  from gluonts.dataset.common import ListDataset
  from autogluon.timeseries.dataset import TimeSeriesDataFrame
  from autogluon.timeseries.dataset.ts_dataframe import ITEMID, TIMESTAMP
  from autogluon.timeseries.metrics import TimeSeriesScorer
label_desc: Use image node_id 2 for calling the CustomMetric obj's underlying member method code with example usage: obj.compute_metric(data_future, predictions, target) and returns: unknown