class_docstr: Trigger based on the starting point of the iteration.
  
  This trigger accepts only once at starting point of the iteration. There
  are two ways to specify the starting point: only starting point in whole
  iteration or called again when training resumed.
  
  Args:
      call_on_resume (bool): Whether the extension is called again or not
          when restored from a snapshot. It is set to ``False`` by default.
  
  Attributes:
      finished (bool): Flag that indicates whether or not this trigger will
      fire in the future. This flag is used to determine if the extension
      should be initialized after resume.
code: def finished(self):
      return not (self._flag_first or self._flag_resumed)
  
imports: import warnings
label_desc: Use image node_id 2 for calling the OnceTrigger obj's underlying member method code with example usage: obj.finished() and returns: unknown