class_docstr: Maintain the moving average of summarized tensors in every step,
  by ops added to the collection.
  Note that it only **maintains** the moving averages by updating
  the relevant variables in the graph,
  the actual summary should be done in other callbacks.
  
  This callback is one of the :func:`DEFAULT_CALLBACKS()`.
class_docstr_tok: ["Maintain","the","moving","average","of","summarized","tensors","in","every","step",",","by","ops","added","to","the","collection",".","Note","that","it","only","*","*","maintains","*","*","the","moving","averages","by","updating","the","relevant","variables","in","the","graph",",","the","actual","summary","should","be","done","in","other","callbacks",".","This","callback","is","one","of","the",":","func",":","`","DEFAULT_CALLBACKS","(",")","`","."]
code: def _before_run(self, _):
      if self._train_op is None:
          return self._fetch
  
imports: import numpy
  from collections import deque
  from ..compat import tfv1
  from ..tfutils.common import get_op_tensor_name
  from ..utils import logger
  from ..utils.naming import MOVING_SUMMARY_OPS_KEY
  from .base import Callback
label_desc: Use image node_id 3 for calling the MovingAverageSummary obj's underlying member method code with example usage: obj._before_run(_) and returns: self