code: @Override
      public INDArray toDense() {
          // TODO support view conversion
          INDArray result = Nd4j.zeros(shape());
  
          switch (data().dataType()) {
              case DOUBLE:
                  for (int i = 0; i < length; i++) {
                      int[] idx = getUnderlyingIndicesOf(i).asInt();
                      double value = values.getDouble(i);
                      result.putScalar(idx, value);
                  }
                  break;
              case FLOAT:
                  for (int i = 0; i < length; i++) {
                      int[] idx = getUnderlyingIndicesOf(i).asInt();
                      float value = values.getFloat(i);
                      result.putScalar(idx, value);
                  }
                  break;
              default:
                  throw new UnsupportedOperationException();
          }
          return result;
      }
docs: Converts the sparse ndarray into a dense one
  @return a dense ndarray