docs: public int layerInputSize(int layer) {
          if (layer < 0 || layer > layers.length) {
              throw new IllegalArgumentException("Invalid layer index: " + layer + ". Layer index must be between 0 and "
                      + (layers.length - 1) + " inclusive");
          }
          org.deeplearning4j.nn.conf.layers.Layer conf = layers[layer].conf().getLayer();
          if (conf == null || !(conf instanceof FeedForwardLayer)) {
              return 0;
          }
          FeedForwardLayer ffl = (FeedForwardLayer) conf;
  
          // FIXME: int cast
          return (int) ffl.getNIn();
      }