### 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)

with client_all_dags.session_transaction() as flask_session:
        flask_session[FILTER_STATUS_COOKIE] = status
    resp = client_all_dags.get(
        "dagmodel/autocomplete?query=example_branch_",
        follow_redirects=False,
    )
    check_content_in_response(expected, resp)
    check_content_not_in_response(unexpected, resp)
@pytest.fixture(scope="module")
def user_all_dags_dagruns(acl_app):
    with create_user_scope(
        acl_app,
        username="user_all_dags_dagruns",
        role_name="role_all_dags_dagruns",
        permissions=[
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG_RUN),
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_WEBSITE),
        ],
    ) as user:
        yield user
@pytest.fixture()
def client_all_dags_dagruns(acl_app, user_all_dags_dagruns):
    return client_with_login(
        acl_app,
        username="user_all_dags_dagruns",
        password="user_all_dags_dagruns",
    )
def test_dag_stats_success(client_all_dags_dagruns):