class_docstr: This class handles the parser event from opt_parser.
  When a target statement is matched, an event is triggered,
  we collected necessary information from this event and construct
  the branches as what we want to understand.
code: def output(self):
      return self.new_graph
  
imports: import nnabla
label_desc: Use image node_id 4 for calling the OptimizerBuilder obj's underlying member method code with example usage: obj.output() and returns: self