@Override
      public InputSplit next() {
        if (this.nextSplit == null && !hasNext()) {
          throw new NoSuchElementException();
        }

        final InputSplit tmp = this.nextSplit;
        this.nextSplit = null;
        return tmp;
      }
Returns the {@code InputFormat}. This is only needed because we need to set the input split
assigner on the {@code StreamGraph}.
amoro-format-mixed/amoro-mixed-flink/amoro-mixed-flink-common/src/main/java/org/apache/amoro/flink/table/UnkeyedInputFormatSourceFunction.java