title: How estimator of tensorflow load specific steps of model instead of latest one?
body: <p>We can save many checkpoints of model using Estimator and <code>RunConfig</code>.</p>
  
  <p><code>classifier</code> eval will use the latest step <code>200</code> by default,
   <strong>can I load <code>ckpt-1</code></strong>?</p>
  
  <pre><code>my_checkpointing_config = tf.estimator.RunConfig(
      save_checkpoints_secs = 20*60,  # Save checkpoints every 20 minutes.
      keep_checkpoint_max = 10,       # Retain the 10 most recent checkpoints.
  )
  classifier = tf.estimator.DNNClassifier(
      feature_columns=my_feature_columns,
      hidden_units=[10, 10],
      n_classes=3,
      model_dir='models/iris',
      config=my_checkpointing_config)
  
  
  
  $ ls -1 models/iris
  checkpoint
  events.out.tfevents.timestamp.hostname
  graph.pbtxt
  model.ckpt-1.data-00000-of-00001
  model.ckpt-1.index
  model.ckpt-1.meta
  model.ckpt-200.data-00000-of-00001
  model.ckpt-200.index
  model.ckpt-200.meta
  </code></pre>