@Override
      public void remove() {
        throw new UnsupportedOperationException();
      }
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