void registerNextOperator(OperatorImpl<RM, ?> nextOperator) {
    if (!initialized) {
      throw new IllegalStateException(
          String.format("Attempted to register next operator before initializing operator %s.",
              getOpImplId()));
    }
    this.registeredOperators.add(nextOperator);
    nextOperator.registerPrevOperator(this);
  }
Register an operator that this operator should propagate its results to.

@param nextOperator  the next operator to propagate results to
samza-core/src/main/java/org/apache/samza/operators/impl/OperatorImpl.java