code: def test_checkpoint_callback_on_iteration_end():
      new_checkpoint_callback = callback.CheckpointCallback(frequency=1)
  
      class Model:
          def save(self, file_path):
              pass
  
      model = Model()
  
      new_checkpoint_callback.on_iteration_end(1, model)
  
file_path: opytimizer/tests/opytimizer/utils/test_callback.py
imports: from opytimark.markers.n_dimensional import Sphere
  from opytimizer.core import function
  from opytimizer.optimizers import swarm
  from opytimizer.opytimizer import Opytimizer
  from opytimizer.spaces import search
  from opytimizer.utils import callback
label_desc: Use image node_id 8 for calling a global function with example usage: test_checkpoint_callback_on_iteration_end() without return types