Question about loss calculation in AR model without handling mask

Regarding the loss calculation part of the AR model, why isn't the mask being handled?
```
total_loss = F.cross_entropy(logits, targets, reduction=reduction)
```
Normally, shouldn't it be:
```
total_loss = F.cross_entropy(logits.mask_selected(y_mask), targets.mask_selected(y_mask), reduction=reduction)
```
What's the reason for not considering the mask?
