Update Trainer code example
# What does this PR do?

This PR updates the code example used in the Trainer docs. Previously, it shows how to overwrite the Trainer to do multi-label classification. However, this is not required anymore, as users can now pass the `problem_type` argument to the model's configuration (to use the appropriate loss function).

Instead, I show how to overwrite the Trainer to use a weighted loss, useful when you have an imbalanced dataset.

Great job merging this PR! the documentation will now be removed from the staging environment.