code: public static String base64StringNumpy(INDArray arr) throws IOException  {
          byte[] bytes = Nd4j.toNpyByteArray(arr);
          return Base64.encodeBase64String(bytes);
      }
docs: Convert an {@link INDArray}
  to numpy byte array using
  {@link Nd4j#toNpyByteArray(INDArray)}
  @param arr the input array
  @return the base 64ed binary
  @throws IOException