Reorder auc only when needed (#91)

* reorder fpr array when needed

* add todo for adding stable sort after pytorch upgrade

* mypy

* set black version to match precommit

anomalib/core/metrics/auroc.py

@@ -1,4 +1,5 @@
 """Implementation of AUROC metric based on TorchMetrics."""
+import torch
 from torch import Tensor
 from torchmetrics import ROC
 from torchmetrics.functional import auc
@@ -13,5 +14,11 @@ def compute(self) -> Tensor:
         Returns:
             Value of the AUROC metric
         """
+        tpr: Tensor
+        fpr: Tensor
+
         fpr, tpr, _thresholds = super().compute()
+        # TODO: use stable sort after upgrading to pytorch 1.9.x (https://github.com/openvinotoolkit/anomalib/issues/92)
+        if not (torch.all(fpr.diff() <= 0) or torch.all(fpr.diff() >= 0)):
+            return auc(fpr, tpr, reorder=True)  # only reorder if fpr is not increasing or decreasing
         return auc(fpr, tpr)