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 __call__(self, trainer):
      fire = not self.finished
      self._flag_resumed = False
      self._flag_first = False
      return fire
  
imports: import warnings
label_desc: Use image node_id 3 for calling the OnceTrigger obj's underlying member method code with example usage: obj.__call__(trainer) and returns: fire