to_categorical should go before get_num_classes in metrics/functional/classification.py

[pwwang]

https://github.com/PyTorchLightning/pytorch-lightning/blob/d18b9ef9d95ffd92591f767808cc497af5bd4e1c/pytorch_lightning/metrics/functional/classification.py#L174-L178

to_categorical should go before get_num_classes, since get_num_classes assumes pred has already been turned into categories (it is doing int(pred.max().detach().item() + 1)).

Warnings are raised now and then with current code, for example:

UserWarning: You have set 10 number of classes if different from predicted (xx) and target (10) number of classes

If logits are passed in directly for metrics.

[github-actions]

Hi! thanks for your contribution!, great first issue!

[Borda]

@pwwang mind send a PR? 🐰