Margin sampling fails with a bad error message if there is only one class in the predictions

[AlexandreAbraham]

We should not expect the classifier to return at least several proba. The error is:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-30-2b90468e1fd1> in <module>
      4                                   n_init_points,
      5                                   base_model,
----> 6                                   semi_model)
<ipython-input-25-008649b258dd> in run_experiment(X_train, X_test, y_train, y_test, batch_size, n_iter, n_init_points, base_model, semi_model)
    118                 sampler.fit(X_train[selected], y_train[selected])
    119 
--> 120                 new_selected = sampler.select_samples(X_train[~selected])
    121                 new_selected = new_selected.astype(int)
    122                 selected[index[~selected][new_selected]] = True
~/dss/code-envs/python/alssl/lib/python3.6/site-packages/cardinAL/base.py in select_samples(self, X, strategy)
     29 
     30     def select_samples(self, X, strategy='top'):
---> 31         sample_scores = self.score_samples(X)
     32         self.sample_scores_ = sample_scores
     33         if strategy == 'top':
~/dss/code-envs/python/alssl/lib/python3.6/site-packages/cardinAL/uncertainty.py in score_samples(self, X)
    175             predictions (np.array): Returns an array where selected samples are classified as 1.
    176         """
--> 177         return margin_score(self.classifier_, X)
    178 
    179 
~/dss/code-envs/python/alssl/lib/python3.6/site-packages/cardinAL/uncertainty.py in margin_score(classifier, X)
     45     """
     46     classwise_uncertainty = _get_probability_classes(classifier, X)
---> 47     part = np.partition(classwise_uncertainty, -2, axis=1)
     48     margin = 1 - (part[:, -1] - part[:, -2])
     49     return margin
<__array_function__ internals> in partition(*args, **kwargs)
~/dss/code-envs/python/alssl/lib/python3.6/site-packages/numpy/core/fromnumeric.py in partition(a, kth, axis, kind, order)
    744     else:
    745         a = asanyarray(a).copy(order="K")
--> 746     a.partition(kth, axis=axis, kind=kind, order=order)
    747     return a
    748 
ValueError: kth(=-1) out of bounds (1)