function createDNNModel() {
  GLOBAL_MODEL = tf.sequential();
  GLOBAL_MODEL.add(tf.layers.dense({
    inputShape: [game.GAME_STATE.max_card_value],
    units: 20,
    activation: 'relu'
  }));
  GLOBAL_MODEL.add(tf.layers.dense({units: 20, activation: 'relu'}));
  GLOBAL_MODEL.add(tf.layers.dense({units: 1, activation: 'sigmoid'}));
  return GLOBAL_MODEL;
}
Returns a three layer sequential model suitable for predicting win state from
feature representation.  The input shape depends on whether oneHot
representation is used.