`ValueError` raised by `FeatureUnion._set_output` with `FunctionTransform` that outputs a pandas `Series` in scikit-learn version 1.6

### Describe the bug

Hello,

I'm currently working with scikit-learn version 1.6, and I encountered a regression that wasn't present in version 1.4.

The following minimal code computes two features — the cumulative mean of age and weight grouped by id. Each transformation function returns a pandas.Series:



When I run this code with scikit-learn 1.6, I get the following error:

After investigation, I found that the issue occurs because each transformer returns a Series, not a DataFrame. If I update the functions to return DataFrame objects instead, the error disappears.

Interestingly, in scikit-learn 1.4, the same code works correctly even when the functions return Series.


Do you have any explanation for why this changed between version 1.4 and 1.6 ?

Thanks in advance for your help!

### Steps/Code to Reproduce


```python
import pandas as pd
from sklearn.pipeline import FunctionTransformer, FeatureUnion
import numpy as np

def compute_cumulative_mean_age(df: pd.DataFrame) -> pd.Series:
    return (
        df["age"]
        .astype(float)
        .groupby(df["id"])
        .expanding()
        .mean()
        .droplevel(level="id")
        .reindex(df.index)
        .rename("cumulative_mean_age")
    )

def compute_cumulative_mean_weight(df: pd.DataFrame) -> pd.Series:
    return (
        df["poids"]
        .astype(float)
        .groupby(df["id"])
        .expanding()
        .mean()
        .droplevel(level="id")
        .reindex(df.index)
        .rename("cumulative_mean_weight")
    )

def compute_features(df: pd.DataFrame) -> pd.DataFrame:
    feature_union = FeatureUnion(
        [
            ("cumulative_mean_age", FunctionTransformer(compute_cumulative_mean_age)),
            ("cumulative_mean_weight", FunctionTransformer(compute_cumulative_mean_weight))
        ]
    ).set_output(transform="pandas")

    return feature_union.fit_transform(X=df).astype(float)

def transform(df: pd.DataFrame) -> pd.DataFrame:
    return compute_features(df)

if __name__ == "__main__":
    np.random.seed(42)
    df = pd.DataFrame({
        'id': [1, 2, 3, 1, 4, 5, 6, 6, 7, 8],
        'age': np.random.randint(18, 70, size=10),
        'poids': np.random.randint(50, 100, size=10)
    })

    print(transform(df))
```

### Expected Results

```pytb
Traceback (most recent call last):
  File "C:\Users\XXX\PycharmProjects\fraude_detection_pec_audio\tmp.py", line 73, in <module>
    print(transform(df=df))
          ^^^^^^^^^^^^^^^^
  File "C:\Users\XXX\PycharmProjects\fraude_detection_pec_audio\tmp.py", line 55, in transform
    return compute_features(
           ^^^^^^^^^^^^^^^^^
  File "C:\Users\XXX\PycharmProjects\fraude_detection_pec_audio\tmp.py", line 45, in compute_features
    .fit_transform(
     ^^^^^^^^^^^^^^
  File "C:\Users\XXX\PycharmProjects\fraude_detection_pec_audio\.venv\Lib\site-packages\sklearn\utils\_set_output.py", line 332, in wrapped
    return _wrap_data_with_container(method, data_to_wrap, X, self)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\XXX\PycharmProjects\fraude_detection_pec_audio\.venv\Lib\site-packages\sklearn\utils\_set_output.py", line 307, in _wrap_data_with_container
    return adapter.create_container(
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\XXX\PycharmProjects\fraude_detection_pec_audio\.venv\Lib\site-packages\sklearn\utils\_set_output.py", line 135, in create_container
    X_output = _create_pandas_dataframe_from_non_pandas_container(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\XXX\PycharmProjects\fraude_detection_pec_audio\.venv\Lib\site-packages\sklearn\utils\fixes.py", line 428, in _create_pandas_dataframe_from_non_pandas_container
    return pd.DataFrame(X, index=index, copy=copy)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\XXX\PycharmProjects\fraude_detection_pec_audio\.venv\Lib\site-packages\pandas\core\frame.py", line 722, in __init__
    mgr = ndarray_to_mgr(
          ^^^^^^^^^^^^^^^
  File "C:\Users\XXX\PycharmProjects\fraude_detection_pec_audio\.venv\Lib\site-packages\pandas\core\internals\construction.py", line 349, in ndarray_to_mgr
    _check_values_indices_shape_match(values, index, columns)
  File "C:\Users\XXX\PycharmProjects\fraude_detection_pec_audio\.venv\Lib\site-packages\pandas\core\internals\construction.py", line 420, in _check_values_indices_shape_match
    raise ValueError(f"Shape of passed values is {passed}, indices imply {implied}")
ValueError: Shape of passed values is (20, 1), indices imply (10, 1)
```

### Actual Results

```pytb
    raise ValueError(f"Shape of passed values is {passed}, indices imply {implied}")
ValueError: Shape of passed values is (20, 1), indices imply (10, 1)
```

### Versions

```shell
sklearn: 1.6.0
numpy: 1.26.4
pandas: 1.5.3
```

COMMENT:
Hi!  @MohammedSaim-Quadri 👋 Thanks for reaching out — and welcome to open source! I'd be more than happy to collaborate on this issue or any other issue. Let’s work on it together and split things up if needed. Feel free to share your thoughts or let me know if you’ve already started exploring a solution. Looking forward to working with you!

Labels: Bug, good first issue