docs: @Override
      public INDArray getParam(String param) {
          //Get params for MultiLayerNetwork sub layers.
          int idx = param.indexOf('_');
          if (idx == -1)
              throw new IllegalStateException("Invalid param key: does not have layer separator: \"" + param + "\"");
          int layerIdx = Integer.parseInt(param.substring(0, idx));
          String newKey = param.substring(idx + 1);
  
          return layers[layerIdx].getParam(newKey);
      }