### What do you see as an issue?


The [documentation for Dynamic Task Mapping](https://airflow.apache.org/docs/apache-airflow/stable/authoring-and-scheduling/dynamic-task-mapping.html#simple-mapping
) does not include an example of a "reduce" task (e.g. `sum_it` in the [examples](https://airflow.apache.org/docs/apache-airflow/stable/authoring-and-scheduling/dynamic-task-mapping.html#simple-mapping)) using the classic (or non-TaskFlow) operators. It only includes an example that uses the TaskFlow operators.

When I attempted to write a "reduce" task using classic operators for my DAG, I found that there wasn't an obvious approach.

### Solving the problem

We should add an example of a "reduce" task that uses the classic (non-TaskFlow) operators.

For example, for the given `sum_it` example:

```
"""Example DAG demonstrating the usage of dynamic task mapping reduce using classic
operators.
"""
from __future__ import annotations

from datetime import datetime

from airflow import DAG
from airflow.decorators import task
from airflow.operators.python import PythonOperator


def add_one(x: int):
    return x + 1

def sum_it(values):
    total = sum(values)
    print(f"Total was {total}")


with DAG(dag_id="example_dynamic_task_mapping_reduce", start_date=datetime(2022, 3, 4)):
    add_one_task = PythonOperator.partial(
        task_id="add_one",
        python_callable=add_one,
    ).expand(
        op_kwargs=[
            {"x": 1},
            {"x": 2},
            {"x": 3},
        ]
    )

    sum_it_task = PythonOperator(
        task_id="sum_it",
        python_callable=sum_it,
        op_kwargs={"values": add_one_task.output},
    )
```

### Anything else

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

"""OperatorLink not registered via Plugins nor a built-in OperatorLink"""
            name = "My Link"
            def get_link(self, operator, *, ti_key):
                return "https://www.google.com"
        class MyOperator(BaseOperator):
            """Just a EmptyOperator using above defined Extra Operator Link"""
            operator_extra_links = [TaskStateLink()]
            def execute(self, context: Context):
                pass
        with DAG(dag_id="simple_dag", start_date=datetime(2019, 8, 1)) as dag:
            MyOperator(task_id="blah")
        serialized_dag = SerializedDAG.to_dict(dag)
        with caplog.at_level("ERROR", logger="airflow.serialization.serialized_objects"):
            SerializedDAG.from_dict(serialized_dag)
        expected_err_msg = (
            "Operator Link class 'tests.serialization.test_dag_serialization.TaskStateLink' not registered"
        )
        assert expected_err_msg in caplog.text
    class ClassWithCustomAttributes:
        """
        Class for testing purpose: allows to create objects with custom attributes in one single statement.
        """
        def __init__(self, **kwargs):
            for key, value in kwargs.items():
                setattr(self, key, value)
        def __str__(self):
            return f"{self.__class__.__name__}({str(self.__dict__)})"
        def __repr__(self):
            return self.__str__()
        def __eq__(self, other):