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 __call__(self, items: List[Item]) -> List[Item]:
      return [
          item
          for item in items
          if len(
              set(
                  item[FieldName.TARGET][
                      -self.required_length : -self.prediction_length
                  ]
              )
          )
          > 1
      ]
  
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 2 for calling the ConstantTargetFilter obj's underlying member method code with example usage: obj.__call__(items) and returns: unknown