class_docstr: Interface for a subnetwork builder.
  
  Given features, labels, and the best ensemble of subnetworks at iteration
  t-1, a `Builder` creates a `Subnetwork` to add to a candidate
  ensemble at iteration t. These candidate ensembles are evaluated against one
  another at the end of the iteration, and the best one is selected based on its
  complexity-regularized loss.
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docstr: Returns a `subnetwork.Report` to materialize and record.
  
  This method will be called once after :meth:`build_subnetwork`.
  Do NOT depend on variables created in :meth:`build_subnetwork_train_op`,
  because they are not called before :meth:`build_subnetwork_report` is
  called.
  
  If it returns None, AdaNet records the name and standard eval metrics.
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code: def build_subnetwork_report(self):
      """Returns a `subnetwork.Report` to materialize and record.
  
      This method will be called once after :meth:`build_subnetwork`.
      Do NOT depend on variables created in :meth:`build_subnetwork_train_op`,
      because they are not called before :meth:`build_subnetwork_report` is
      called.
  
      If it returns None, AdaNet records the name and standard eval metrics.
      """
  
      return None
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import abc
  import collections
  import six
label_desc: Use image node_id 4 for calling the Builder obj's underlying member method code with example usage: obj.build_subnetwork_report() and returns: None