### 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)

tg.expand(a=[".", ".."])
    ser_dag = SerializedBaseOperator.serialize(dag)
    assert ser_dag["_task_group"]["children"]["tg"] == (
        "taskgroup",
        {
            "_group_id": "tg",
            "children": {
                "tg.op1": ("operator", "tg.op1"),
                # "tg.op2": ("operator", "tg.op2"),
            },
            "downstream_group_ids": [],
            "downstream_task_ids": [],
            "expand_input": {
                "type": "dict-of-lists",
                "value": {"__type": "dict", "__var": {"a": [".", ".."]}},
            },
            "is_mapped": True,
            "prefix_group_id": True,
            "tooltip": "",
            "ui_color": "CornflowerBlue",
            "ui_fgcolor": "#000",
            "upstream_group_ids": [],
            "upstream_task_ids": [],
        },
    )
    serde_dag = SerializedDAG.deserialize_dag(ser_dag)
    serde_tg = serde_dag.task_group.children["tg"]
    assert isinstance(serde_tg, MappedTaskGroup)
    assert serde_tg._expand_input == DictOfListsExpandInput({"a": [".", ".."]})