async loadModel(lstmLayerSizes) {
    const modelsInfo = await tf.io.listModels();
    if (this.modelSavePath_ in modelsInfo) {
      console.log(`Loading existing model...`);
      this.model = await tf.loadLayersModel(this.modelSavePath_);
      console.log(`Loaded model from ${this.modelSavePath_}`);
    } else {
      throw new Error(
          `Cannot find model at ${this.modelSavePath_}. ` +
          `Creating model from scratch.`);
    }
  }
Create LSTM model if it is not saved locally; load it if it is.

@param {number | number[]} lstmLayerSizes Sizes of the LSTM layers, as a
  number or an non-empty array of numbers.