Improve OptunaSearchCV (accept Mapping in addition to dict for param_distributions and allow use of OptunaSearchCV with cross_val_predict)

[yu9824]

Motivation

I have two motivations.

1. Accept Mapping in addition to dict for param_distributions.
2. Allow use of OptunaSearchCV with cross_val_predict.

Description of the changes

1. Accept Mapping in addition to dict for param_distributions.

Changed the accepted type from dict to Mapping.

MappingProxyType and other types cannot be made to pickle, which causes an error during parallelization, so the specification has been changed to change to dict after accepting the Mapping type.

2. Allow use of OptunaSearchCV with cross_val_predict.

In scikit-learn's HasMethods, predict (and related methods) get error because of 'property', so define them as method.

[y0z]

@nabenabe0928 Could you review this PR?

[nabenabe0928]

Could you first move back each method to the original location?
Currently, this PR has much more diffs than we need.

Plus, could you give me the steps to reproduce your error?

[nabenabe0928]

@yu9824

So, I, first of all, found the exceptions where your changes do not work:

• https://scikit-learn.org/stable/modules/generated/sklearn.tree.DecisionTreeClassifier.html#sklearn.tree.DecisionTreeClassifier.predict_proba
• https://scikit-learn.org/stable/modules/generated/sklearn.tree.ExtraTreeClassifier.html#sklearn.tree.ExtraTreeClassifier.predict_proba

It means we cannot merge this PR as is, so I will close this PR for now.
However, please feel free to open this PR again if you find any other solutions!
Or you can alternatively post an issue for this regard with the steps to reproduce.

[yu9824]

Thank you for your review and comment.

I'll create issues alternatively to show you how to reproduce errors.
And, I'll deal with your comment.

[yu9824]

I've created new issues (#171, #173) and PRs (#172, #174).

I cannot reproduce your error below. Please tell me in the new PRs.

@yu9824

So, I, first of all, found the exceptions where your changes do not work:

• https://scikit-learn.org/stable/modules/generated/sklearn.tree.DecisionTreeClassifier.html#sklearn.tree.DecisionTreeClassifier.predict_proba
• https://scikit-learn.org/stable/modules/generated/sklearn.tree.ExtraTreeClassifier.html#sklearn.tree.ExtraTreeClassifier.predict_proba

It means we cannot merge this PR as is, so I will close this PR for now. However, please feel free to open this PR again if you find any other solutions! Or you can alternatively post an issue for this regard with the steps to reproduce.

I appreciate you taking the time to review this.