class_docstr: A CheckpointCallback class that handles additional logging and
  model's checkpointing.
docstr: Interval between checkpoints.
docstr_tok: ["Interval","between","checkpoints","."]
code: def frequency(self) -> int:
      """Interval between checkpoints."""
  
      return self._frequency
  
imports: from typing import List, TypeVar, Union
  import numpy
  import opytimizer.utils.exception
  from opytimizer.core.space import Space
label_desc: Use image node_id 4 for calling the CheckpointCallback obj's underlying member method code with example usage: obj.frequency() and returns: self