hotfix on classification metrics (#2878)

* Faster classfication stats

* Faster accuracy metric

* minor change on cls metric

* Add out-of-bound class clamping

* Add more tests and minor fixes

* Resolve code style warning

* Update for #2781

* hotfix

* Update pytorch_lightning/metrics/functional/classification.py

Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>

* Update about conversation

* Add docstring on stat_scores_multiple_classes

* Fixing #2862

Co-authored-by: Younghun Roh <yhunroh@mindslab.ai>
Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>

pytorch_lightning/metrics/functional/classification.py

@@ -181,9 +181,9 @@ def stat_scores_multiple_classes(
     num_classes = get_num_classes(pred=pred, target=target, num_classes=num_classes)
 
     if pred.dtype != torch.bool:
-        pred.clamp_max_(max=num_classes)
+        pred = pred.clamp_max(max=num_classes)
     if target.dtype != torch.bool:
-        target.clamp_max_(max=num_classes)
+        target = target.clamp_max(max=num_classes)
 
     possible_reductions = ('none', 'sum', 'elementwise_mean')
     if reduction not in possible_reductions: