class_docstr: Trainer extension to unchain all comptational graphs.
  
  This extenstion unchains all comptational graphs after all extensions are
  run to release memory and to avoid memory leak.
  This extension can be used as a last resort when there is an extension that
  use a variable graph and cannot release the graph in itself.
  It observes the previous ``chainer.config.keep_graph_on_report`` flag.
  The extension is triggered when the flag is turned on.
code: def initialize(self, _):
      self._prev_flag = configuration.config.keep_graph_on_report
  
file_path: chainer/chainer/training/extensions/variable_unchain.py
imports: import six
  from chainer import configuration
  from chainer.training import extension
  from chainer import variable
label_desc: Use image node_id 2 for calling the unchain_variables obj's underlying member method code with example usage: obj.initialize(_) without return types