### 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)

), f"{task.dag.dag_id}.{task.task_id}.{field} does not match"
        if serialized_task.resources is None:
            assert task.resources is None or task.resources == []
        else:
            assert serialized_task.resources == task.resources
        
        if isinstance(task.params, ParamsDict) and isinstance(serialized_task.params, ParamsDict):
            assert serialized_task.params.dump() == task.params.dump()
        if isinstance(task, MappedOperator):
            
            
            
            serialized_task.operator_class["_task_type"] == type(task).__name__
            serialized_task.operator_class["_operator_name"] == task._operator_name
            
            
            default_partial_kwargs = (
                BaseOperator.partial(task_id="_")._expand(EXPAND_INPUT_EMPTY, strict=False).partial_kwargs
            )
            serialized_partial_kwargs = {**default_partial_kwargs, **serialized_task.partial_kwargs}
            original_partial_kwargs = {**default_partial_kwargs, **task.partial_kwargs}
            assert serialized_partial_kwargs == original_partial_kwargs
        
        
        if task.task_type == "SubDagOperator":
            assert serialized_task.subdag is not None
            assert isinstance(serialized_task.subdag, DAG)
        else:
            assert serialized_task.subdag is None
    @pytest.mark.parametrize(