### 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.decorators import task
    from airflow.models.xcom_arg import XComArg
    from airflow.serialization.serialized_objects import _ExpandInputRef, _XComRef
    with DAG("test-dag", start_date=datetime(2020, 1, 1)) as dag:
        op1 = BaseOperator(task_id="op1")
        @task(retry_delay=30)
        def x(arg1, arg2, arg3):
            print(arg1, arg2, arg3)
        x.partial(arg1=[1, 2, {"a": "b"}]).expand_kwargs(XComArg(op1), strict=strict)
    original = dag.get_task("x")
    serialized = SerializedBaseOperator.serialize(original)
    assert serialized == {
        "_is_empty": False,
        "_is_mapped": True,
        "_task_module": "airflow.decorators.python",
        "_task_type": "_PythonDecoratedOperator",
        "_operator_name": "@task",
        "downstream_task_ids": [],
        "partial_kwargs": {
            "op_args": [],
            "op_kwargs": {
                "__type": "dict",
                "__var": {"arg1": [1, 2, {"__type": "dict", "__var": {"a": "b"}}]},