Describe the issue linked to the documentation

Hi,

We discover an inconsistency issue between documentation and code in the class sklearn.compose.TransformedTargetRegressor. As mentioned in the description of parameter func.

func: function, default=None
Function to apply to y before passing to fit. Cannot be set at the same time as transformer. The function needs to return a 2-dimensional array. If func is None, the function used will be the identity function.

The most relevant piece of source code looks like this:

if self.func is not None and self.inverse_func is None:
    raise ValueError(
        "When 'func' is provided, 'inverse_func' must also be provided"
    )

The error in the code mentioned that when func is provided, inverse_func must also be provided. The constraint is not mentioned in the document.

Could you please check it?

Suggest a potential alternative/fix

Maybe you can add the constraint into the document to avoid unnecessary misuse and extra debug efforts.