[Research] `SVC`, `SVR` expose `n_iter_`, could be added to iterative fit

[eddiebergman]

Enhancement svm.OneClassSVM, svm.NuSVC, svm.NuSVR, svm.SVC and svm.SVR now expose n_iter_, the number of iterations of the libsvm optimization routine. scikit-learn/scikit-learn#21408 by Juan Martín Loyola.

Mentioned in #1611

With this parameter exposed, we might be able to shove these into the category of iterable_fit style methods.