### 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)

"""
        This test verifies that there are no new fields added to BaseOperator. And reminds that
        tests should be added for it.
        """
        base_operator = BaseOperator(task_id="10")
        fields = {k: v for (k, v) in vars(base_operator).items() if k in BaseOperator.get_serialized_fields()}
        assert fields == {
            "_log": base_operator.log,
            "_post_execute_hook": None,
            "_pre_execute_hook": None,
            "depends_on_past": False,
            "do_xcom_push": True,
            "doc": None,
            "doc_json": None,
            "doc_md": None,
            "doc_rst": None,
            "doc_yaml": None,
            "downstream_task_ids": set(),
            "email": None,
            "email_on_failure": True,
            "email_on_retry": True,
            "execution_timeout": None,
            "executor_config": {},
            "ignore_first_depends_on_past": True,
            "inlets": [],
            "max_active_tis_per_dag": None,
            "max_active_tis_per_dagrun": None,
            "max_retry_delay": None,
            "on_execute_callback": None,
            "on_failure_callback": None,