class_docstr: Base class for all callbacks. See
  `Write a Callback
  <http://tensorpack.readthedocs.io/tutorial/extend/callback.html>`_
  for more detailed explanation of the callback methods.
  
  Attributes:
      epoch_num(int): trainer.epoch_num
      global_step(int): trainer.global_step
      local_step(int): trainer.local_step
      trainer(Trainer): the trainer.
      graph(tf.Graph): the graph.
  
  Note:
      These attributes are available only after (and including)
      :meth:`_setup_graph`.
  
  .. document private functions
  .. automethod:: _setup_graph
  .. automethod:: _before_train
  .. automethod:: _after_train
  .. automethod:: _before_run
  .. automethod:: _after_run
  .. automethod:: _before_epoch
  .. automethod:: _after_epoch
  .. automethod:: _trigger_step
  .. automethod:: _trigger_epoch
  .. automethod:: _trigger
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code: def trigger_epoch(self):
      self._trigger_epoch()
  
imports: from abc import ABCMeta
  import six
  from ..compat import tfv1
  from ..tfutils.common import get_op_or_tensor_by_name
label_desc: Use image node_id 15 for calling the Callback obj's underlying member method code with example usage: obj.trigger_epoch() without return types