class_docstr: A filter which removes items having only constant target values. This filter should be used
  whenever metrics such as the MASE are required.
code: def __init__(self, prediction_length: int, required_length: int = 0):
      self.prediction_length = prediction_length
      self.required_length = required_length
  
imports: from abc import ABC, abstractmethod
  from typing import Any, Dict, List
  import numpy
  from gluonts.dataset.field_names import FieldName
label_desc: Use image node_id 1 to create a new ConstantTargetFilter object from inherited base classes: Filter with example: obj = ConstantTargetFilter(prediction_length, required_length)