Update auc.py

Not sure should it be more efficient leaving `torch.unique` at line #50 there or not.

allennlp/training/metrics/auc.py

@@ -52,7 +52,7 @@ def __call__(self,
             raise ConfigurationError("AUC can be used for binary tasks only. gold_labels has {} unique labels, "
                                      "expected at maximum 2.".format(unique_gold_labels.numel()))
 
-        gold_labels_is_binary = set(torch.sort(unique_gold_labels)[0].numpy()) <= set([0, 1])
+        gold_labels_is_binary = set(unique_gold_labels.tolist()) <= {0, 1}
         if not gold_labels_is_binary and self._positive_label not in unique_gold_labels:
             raise ConfigurationError("gold_labels should be binary with 0 and 1 or initialized positive_label "
                                      "{} should be present in gold_labels".format(self._positive_label))