docs: public void fit(GraphWalkIterator<V> iterator) {
          if (!initCalled)
              throw new UnsupportedOperationException("DeepWalk not initialized (call initialize before fit)");
          int walkLength = iterator.walkLength();
  
          while (iterator.hasNext()) {
              IVertexSequence<V> sequence = iterator.next();
  
              //Skipgram model:
              int[] walk = new int[walkLength + 1];
              int i = 0;
              while (sequence.hasNext())
                  walk[i++] = sequence.next().vertexID();
  
              skipGram(walk);
  
              long iter = walkCounter.incrementAndGet();
              if (iter % STATUS_UPDATE_FREQUENCY == 0) {
                  log.info("Processed {} random walks on graph", iter);
              }
          }
      }