try to fix bug concerning input

horizon-metrics.py

@@ -177,8 +177,8 @@ class HorizonMetrics(evaluate.Metric):
         """
 
         if (self.ground_truth_det is None or self.predictions is None):
-            self.ground_truth_det = references
-            self.predictions = predictions
+            self.ground_truth_det = references[0]
+            self.predictions = predictions[0]
             print(self.ground_truth_det)
             print(self.predictions)