code: public Map<String, INDArray> rnnGetPreviousState(int layer) {
          return rnnGetPreviousState(layers[layer].conf().getLayer().getLayerName());
      }
docs: Get the state of the RNN layer, as used in {@link #rnnTimeStep(INDArray...)}.
  
  @param layer Number/index of the layer.
  @return Hidden state, or null if layer is not an RNN layer