safeguard for oversampling strategy, when DS is negatives only or sin…

…gle pos pair

src/setfit/sampler.py

@@ -129,8 +129,10 @@ def __init__(
             self.len_neg_pairs = min([min(self.pos_pairs_combination, self.neg_pairs_combination), self.max_pos_or_neg])
 
         elif sampling_strategy == SamplingStrategy.OVERSAMPLING:
-            self.len_pos_pairs = min([max(self.pos_pairs_combination, self.neg_pairs_combination), self.max_pos_or_neg])
-            self.len_neg_pairs = min([max(self.pos_pairs_combination, self.neg_pairs_combination), self.max_pos_or_neg])
+            num_pos_or_neg_pairs = max(self.pos_pairs_combination, self.neg_pairs_combination)
+            # saveguard for either negative samples only or single positive.
+            self.len_pos_pairs = min([num_pos_or_neg_pairs, self.max_pos_or_neg]) if self.pos_pairs_combination > 0 else 0
+            self.len_neg_pairs = min([num_pos_or_neg_pairs, self.max_pos_or_neg]) if self.neg_pairs_combination > 0 else 0
 
     def generate_positive_pair(self) -> Generator[SentencePair, None, None]:
         pair_generator = shuffle_combinations(self.sentence_labels)