support class probability output for SVM classifiers

[yitao-li]

RAPIDS cuML, similar to scikit-learn, by default uses 5-fold cross validation to calibrate class probabilities: see https://github.com/rapidsai/cuml/blob/57a6ff7ecae6a4b6e2e9db97dc664be453310111/python/cuml/svm/svc.pyx#L394-L396