message: Explain some magic in freeze_graph_tpu
diff: @@ -547,7 +547,12 @@ def freeze_graph_tpu(model_path):
  tf.identity(policy_output, policy_name)
  tf.identity(value_output, value_name)
  - # Add initialize and shutdown TPU ops.
  + # Add initialize and shutdown TPU ops to the graph.
  + # The ops aren't actually executed here. Instead, the serialized ops are
  + # run by the C++ TpuDualNet implementation to perform one-time
  + # initialization and shutdown of the TPU. We do it this way because
  + # TensorFlow currently doesn't currently expose a C++ API for TPU
  + # initialization and shutdown.
  init_def = tf.contrib.tpu.initialize_system()
  shutdown_def = tf.contrib.tpu.shutdown_system()
  