code: def add_model_variable(var):
      """Adds a variable to the `GraphKeys.MODEL_VARIABLES` collection.
  
      Args:
        var: a variable.
      """
      if var not in ops.get_collection(ops.GraphKeys.MODEL_VARIABLES):
          ops.add_to_collection(ops.GraphKeys.MODEL_VARIABLES, var)
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import functools
  import re
  from .arg_scope import add_arg_scope
  from tensorflow.core.protobuf import saver_pb2
  from tensorflow.python import pywrap_tensorflow
  from tensorflow.python.framework import device
  from tensorflow.python.framework import dtypes
  from tensorflow.python.framework import ops
  from tensorflow.python.ops import array_ops
  from tensorflow.python.ops import control_flow_ops
  from tensorflow.python.ops import resource_variable_ops
  from tensorflow.python.ops import variable_scope
  from tensorflow.python.ops import variables
  from tensorflow.python.platform import resource_loader
  from tensorflow.python.platform import tf_logging
  from tensorflow.python.training import saver
  from tensorflow.python.training import training_util
  from tensorflow.python.util.deprecation import deprecated
label_desc: Use image node_id 10 for calling a global function with example usage: add_model_variable(var) without return types