### 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)

from airflow.kubernetes.pod_generator import PodGenerator
from airflow.models import DAG, Connection, DagBag, Operator
from airflow.models.baseoperator import BaseOperator, BaseOperatorLink
from airflow.models.expandinput import EXPAND_INPUT_EMPTY
from airflow.models.mappedoperator import MappedOperator
from airflow.models.param import Param, ParamsDict
from airflow.models.xcom import XCom
from airflow.operators.bash import BashOperator
from airflow.operators.empty import EmptyOperator
from airflow.security import permissions
from airflow.sensors.bash import BashSensor
from airflow.serialization.json_schema import load_dag_schema_dict
from airflow.serialization.serialized_objects import (
    DagDependency,
    DependencyDetector,
    SerializedBaseOperator,
    SerializedDAG,
)
from airflow.ti_deps.deps.base_ti_dep import BaseTIDep
from airflow.timetables.simple import NullTimetable, OnceTimetable
from airflow.utils import timezone
from airflow.utils.context import Context
from airflow.utils.operator_resources import Resources
from airflow.utils.task_group import TaskGroup
from airflow.utils.xcom import XCOM_RETURN_KEY
from tests.test_utils.config import conf_vars
from tests.test_utils.mock_operators import CustomOperator, GoogleLink, MockOperator
from tests.test_utils.timetables import CustomSerializationTimetable, cron_timetable, delta_timetable
repo_root = Path(airflow.__file__).parent.parent
class CustomDepOperator(BashOperator):