Update Trainer code example (#15070)

* Update code example

* Fix code quality

* Add comment

docs/source/main_classes/trainer.mdx

@@ -47,22 +47,22 @@ when you use it on other models. When using it on your own model, make sure:
 
 </Tip>
 
-Here is an example of how to customize [`Trainer`] using a custom loss function for multi-label classification:
+Here is an example of how to customize [`Trainer`] to use a weighted loss (useful when you have an unbalanced training set):
 
 ```python
 from torch import nn
 from transformers import Trainer
 
 
-class MultilabelTrainer(Trainer):
+class CustomTrainer(Trainer):
     def compute_loss(self, model, inputs, return_outputs=False):
         labels = inputs.get("labels")
+        # forward pass
         outputs = model(**inputs)
         logits = outputs.get("logits")
-        loss_fct = nn.BCEWithLogitsLoss()
-        loss = loss_fct(
-            logits.view(-1, self.model.config.num_labels), labels.float().view(-1, self.model.config.num_labels)
-        )
+        # compute custom loss (suppose one has 3 labels with different weights)
+        loss_fct = nn.CrossEntropyLoss(weight=torch.tensor([1.0, 2.0, 3.0]))
+        loss = loss_fct(logits.view(-1, self.model.config.num_labels), labels.view(-1))
         return (loss, outputs) if return_outputs else loss
 ```