static async loadModel() {
    const modelsInfo = await tf.io.listModels();
    if (MODEL_SAVE_PATH_ in modelsInfo) {
      console.log(`Loading existing model...`);
      const model = await tf.loadLayersModel(MODEL_SAVE_PATH_);
      console.log(`Loaded model from ${MODEL_SAVE_PATH_}`);
      return new SaveablePolicyNetwork(model);
    } else {
      throw new Error(`Cannot find model at ${MODEL_SAVE_PATH_}.`);
    }
  }
Load the model from IndexedDB.

@returns {SaveablePolicyNetwork} The instance of loaded
  `SaveablePolicyNetwork`.
@throws {Error} If no model can be found in IndexedDB.