class_docstr: Trigger invoked at specified point(s) of iterations or epochs.
  
  This trigger accepts iterations or epochs indicated by given point(s).
  There are two ways to specify the point(s): iteration and epoch.
  ``iteration`` means the number of updates, while ``epoch`` means the number
  of sweeps over the training dataset. Fractional values are allowed
  if the point is a number of epochs; the trigger uses the ``iteration``
  and ``epoch_detail`` attributes defined by the updater.
  
  Args:
      points (int, float, or list of int or float): time of the trigger.
          Must be an integer or list of integer if unit is ``'iteration'``.
      unit (str): Unit of the time specified by ``points``. It must be
          either ``'iteration'`` or ``'epoch'``.
  
  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.
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code: def serialize(self, serializer):
      try:
          self._previous_iteration = serializer(
              "previous_iteration", self._previous_iteration
          )
      except KeyError:
          warnings.warn(
              "The previous value of iteration is not saved. "
              "ManualScheduleTrigger guesses it using current iteration. "
              "If this trigger is not called at every iteration, "
              "it may not work correctly."
          )
          # set a negative value for invalid
          self._previous_iteration = -1
  
      try:
          self._previous_epoch_detail = serializer(
              "previous_epoch_detail", self._previous_epoch_detail
          )
      except KeyError:
          warnings.warn(
              "The previous value of epoch_detail is not saved. "
              "ManualScheduleTrigger uses the value of "
              "trainer.updater.previous_epoch_detail. "
              "If this trigger is not called at every iteration, "
              "it may not work correctly."
          )
          # set a negative value for invalid
          self._previous_epoch_detail = -1.0
  
      try:
          self.finished = serializer("finished", self.finished)
      except KeyError:
          warnings.warn(
              "The flag of finished is not saved. "
              "ManualScheduleTrigger set the flag to `False` to force "
              "initialization and reset in next `__call__`."
          )
          # set False to force initialization.
          self.finished = False
          self._finished_is_tmp = True
  
imports: import warnings
label_desc: Use image node_id 3 for calling the ManualScheduleTrigger obj's underlying member method code with example usage: obj.serialize(serializer) without return types