[Docs] Fix typo in CustomTrainer compute_loss method and adjust loss reduction logic (#39391)

Fix typo in CustomTrainer compute_loss method and adjust loss reduction logic

docs/source/en/trainer.md

@@ -187,13 +187,13 @@ from torch import nn
 from transformers import Trainer
 
 class CustomTrainer(Trainer):
-    def compute_losss(self, model: nn.Module, inputs: dict[str, Union[torch.Tensor, Any]], return_outputs: bool = False num_items_in_batch: Optional[torch.Tensor] = None):
+    def compute_loss(self, model: nn.Module, inputs: dict[str, Union[torch.Tensor, Any]], return_outputs: bool = False num_items_in_batch: Optional[torch.Tensor] = None):
         labels = inputs.pop("labels")
         # forward pass
         outputs = model(**inputs)
         logits = outputs.get("logits")
         # compute custom loss for 3 labels with different weights
-        reduction = "mean" if num_items_in_batch is not None else "sum"
+        reduction = "sum" if num_items_in_batch is not None else "mean"
         loss_fct = nn.CrossEntropyLoss(weight=torch.tensor([1.0, 2.0, 3.0], device=model.device, reduction=reduction))
         loss = loss_fct(logits.view(-1, self.model.config.num_labels), labels.view(-1))
         if num_items_in_batch is not None: