RFC refactoring of PartialDependenceDisplay

Related to https://github.com/scikit-learn/scikit-learn/issues/15641

I would like to discuss the possibility to refactor `PartialDependenceDisplay` to reduce its usability. The idea behind this refactoring is:

- make it consistent with other displays: calling `PartialDependenceDisplay` is equivalent to a call to `partial_dependence`.
- it will simplify greatly the code base: the codebase was already refactored but this is still difficult to understand logic where axes are interacting with each other.
- keep the figure close to standard `matplotlib`: it allows a user that knows `matplotlib` to quickly modify and tune the figure without knowing some implementation details enforced by `scikit-learn`.

### Sharing y-axis

One main reason for having `PartialDependenceDisplay` doing several plots is mainly for setting a common y-axis. However, `matplotlib` allows such a feature when creating a figure:

```python
features = ["age", "sex"]
fig, axs = plt.subplots(figsize=(12, 4), ncols=len(features), sharey=True)
for feat, ax in zip(features, axs):
    pd_values = partial_dependence(
        model,
        X,
        features=feat,
        categorical_features=categorical_features,
    )
    if feat in categorical_features:
        ax.bar(
                pd_values["values"][0], pd_values["average"][0]
            )
    else:
        ax.plot(
            pd_values["values"][0], pd_values["average"][0]
        )
```

![image](https://user-images.githubusercontent.com/7454015/204843253-6983838c-2ce7-494c-98d5-e0a17c2a5048.png)

Here, the call could be done with `PartialDependenceDisplay` instead of the call to `partial_dependence` + plotting.

In this case, a user can still manipulate the difference axes using `axs` and make any usual matplotlib.

### Surprising API

With the current `PartialDependenceDisplay`, we will have some parameters specific to our settings: `ncols` to set the number of columns and fitted attribute `bounding_axis_` and `axes_`. The latest attributes are required if someone is giving an axis, for instance, to provide a certain figure size:

```python
fig, ax = plt.subplots(figsize=(12, 4))
disp = PartialDependenceDisplay.from_estimator(
    ...
    ax=ax,
)
```

I personally find it more intuitive to have `ax` containing all the axes without picking up in the scikit-learn documentation, which way the plot is organized.

### Cons

The only thing that could be more difficult is to plot 1D and 2D PD at the same time. Since one wants to share y-axis for 1D plot, this is indeed not a desired feature to have for the 2D PD. Therefore, it requires our user to know how to share y-axis only for some of the axis.

At the same time, it could be argued that having a mixed type of 1D and 2D plots on the same figure should not be a feature :) Those should be on separate figures.

### Proposed API

The new API would force the user to materialize a `for` loop to handle each individual plot as:

```python
features = ["age", "sex"]
fig, axs = plt.subplots(figsize=(12, 4), ncols=len(features), sharey=True)
for feat, ax in zip(features, axs):
    PartialDependenceDisplay.from_estimator(
        model,
        X,
        feature=feat,
        categorical_features=categorical_features,
        ax=ax,
    )

COMMENT:
> I am not sure I understand the proposal. Is the goal to make PartialDependenceDisplay compute and display one PD plot at a time?

Yes, this is exactly my proposal.

> Would you suggest introducing another class for dedicated for 2-way PD plots and reserve the PartialDependenceDisplay for the one-feature-at-a-time PD-plots?

I think this is fine to have a single class. `partial_dependence` allows to compute interaction and it should be possible to do so with `PartialDependenceDisplay`. Regarding maintenance of the code, we already have 2 branches in the code with one private function to plot 1D PDP and one function to plot 2D PDP. This part of the code is not too much of a hassle to understand.

Labels: RFC, module:inspection