disable checkpointing with pure tf implementation

[psteinb]

the current implementation of the pure tf based resnet taken from here doesn't run with checkpointing disabled. I tried several things following the docs in the spirit of:

run_config = tf.estimator.RunConfig().replace(save_checkpoints_secs=None,
                                                  save_checkpoints_steps=None,
                                                  session_config=session_config)

But always ran into an error.