code: def _save_variables_to_ckpt(model_dir):
      """Save all graph variables in a checkpoint under 'model_dir'."""
      with tf.compat.v1.Session() as sess:
          sess.run(tf.compat.v1.global_variables_initializer())
          tf.compat.v1.train.Saver().save(
              sess, os.path.join(model_dir, "model.ckpt")
          )
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import os
  from absl.testing import parameterized
  import tensorflow
  from tensorflow import estimator
  from tensorflow_ranking.python import feature
  from tensorflow_ranking.python import head
  from tensorflow_ranking.python import losses
  from tensorflow_ranking.python import model
label_desc: Use image node_id 1 for calling a global function with example usage: _save_variables_to_ckpt(model_dir) without return types