title: freeze_graph_with_def_protos doesn't exist
body: <p>I'm currently having an issue from exporting SSD_Model to tflite.
  I'm currently using this command:</p>
  
  <blockquote>
    <p>!python
    /content/models/research/object_detection/export_tflite_ssd_graph.py —
    pipeline_config_path {pipeline_fname} — trained_checkpoint_prefix
    “/content/models/research/training/model.ckpt-1000” — output_directory
    “/content/object_detection_demo/” — add_postprocessing_op True —
    max_detections 10</p>
  </blockquote>
  
  <p>But it returning an error saying</p>
  
  <pre><code>Traceback (most recent call last):
    File "/content/models/research/object_detection/export_tflite_ssd_graph.py", line 143, in &lt;module&gt;
      tf.compat.v1.app.run(main)
    File "/usr/local/lib/python3.6/dist-packages/tensorflow_core/python/platform/app.py", line 40, in run
      _run(main=main, argv=argv, flags_parser=_parse_flags_tolerate_undef)
    File "/usr/local/lib/python3.6/dist-packages/absl/app.py", line 299, in run
      _run_main(main, args)
    File "/usr/local/lib/python3.6/dist-packages/absl/app.py", line 250, in _run_main
      sys.exit(main(argv))
    File "/content/models/research/object_detection/export_tflite_ssd_graph.py", line 139, in main
      FLAGS.max_classes_per_detection, use_regular_nms=FLAGS.use_regular_nms)
    File "/content/models/research/object_detection/export_tflite_ssd_graph_lib.py", line 306, in export_tflite_graph
      initializer_nodes='')
    File "/usr/local/lib/python3.6/dist-packages/tensorflow_core/python/tools/freeze_graph.py", line 129, in freeze_graph_with_def_protos
      "' doesn't exist!")
  ValueError: Input checkpoint '' doesn't exist!
  </code></pre>
  
  <p>Tensorflow version:  1.15.0
  Python version:  3.6.8</p>
  
  <p>TIA</p>