Metrics: AUC

[justusschock]

🚀 Feature

Implement general AUC (to be combined with other metrics like ROC)

[nickyi1990]

just use from sklearn.metrics import roc_auc_score, I don't think there's any need to write this function from scratch

[justusschock]

Actually there is. Using the sklearn function involves the necessity to convert from torch.Tensor to numpy.ndarray before converting. While this is totally fine on cpu (no memory copy made here), this really causes a slow-down for gpu tensors, since it involves a GPU-sync.
For now #1327 already implements a sklearn version, but it shall be replaced with a native torch one.

[justusschock]

@Borda I reopened this, since this is meant for native implementations in #1488