decision_function on the estimator with the best found parameters.

Only available if ``refit=True`` and the underlying estimator supports
``decision_function``.

Parameters
----------
X : indexable, length n_samples
    Must fulfill the input assumptions of the
    underlying estimator.

Returns
-------
y_score : ndarray of shape (n_samples,) or (n_samples, n_classes)                 or (n_samples, n_classes * (n_classes-1) / 2)
    Result of the decision function for `X` based on the estimator with
    the best found parameters.
)