class_docstr: A CustomCallback can be created by override its parent `Callback` class
  and by implementing the desired logic in its available methods.
docstr: Called at the beginning of an task.
code: def on_task_begin(self, opt_model):
      """Called at the beginning of an task."""
  
      pass
  
file_path: opytimizer/examples/utils/custom_callbacks.py
imports: from opytimizer.utils.callback import Callback
label_desc: Use image node_id 2 for calling the CustomCallback obj's underlying member method code with example usage: obj.on_task_begin(opt_model) without return types