class_docstr: Collect all tf.Tensor object, to be evaluated by tf.Session.run()
  These objects are grouped into::
  
      NETWORK_OUTPUT: to be decoded by an aggregator
      CONSOLE: to be printed on command line
      TF_SUMMARIES: to be added to tensorboard visualisation
code: def __init__(self, n_devices=1):
      self.console_vars = {}
      self.summary_vars = {}
      self.output_vars = {}
  
      self._merge_op = None
      self.n_devices = n_devices
  
imports: from __future__ import absolute_import, print_function
  import tensorflow
  from tensorflow.contrib.framework import list_variables
  from niftynet.io.misc_io import image3_axial, image3_coronal, image3_sagittal, resolve_checkpoint
  from niftynet.utilities import util_common
  from niftynet.utilities.restore_initializer import restore_initializer
label_desc: Use image node_id 1 to create a new OutputsCollector object from inherited base classes: object with example: obj = OutputsCollector(n_devices)