hiddenLayerSizes() {
    const sizes = [];
    for (let i = 0; i < this.policyNet.layers.length - 1; ++i) {
      sizes.push(this.policyNet.layers[i].units);
    }
    return sizes.length === 1 ? sizes[0] : sizes;
  }
Get the sizes of the hidden layers.

@returns {number | number[]} If the model has only one hidden layer,
  return the size of the layer as a single number. If the model has
  multiple hidden layers, return the sizes as an Array of numbers.