use mean instead of elementwise_mean in XLMPredLayer
# What does this PR do?

Super tiny change:

`XLMPredLayer` currently uses

```
                loss = nn.functional.cross_entropy(
                    scores.view(-1, self.n_words), y.view(-1), reduction="elementwise_mean"
                )
```
I believe `elementwise_mean` is deprecated:

https://github.com/pytorch/pytorch/pull/13419

https://pytorch.org/docs/stable/generated/torch.nn.functional.cross_entropy.html

_The documentation is not available anymore as the PR was closed or merged._