fix documentation for CustomTrainer (huggingface#25635)

fix doc

docs/source/en/main_classes/trainer.md

@@ -60,7 +60,7 @@ from transformers import Trainer
 
 class CustomTrainer(Trainer):
     def compute_loss(self, model, inputs, return_outputs=False):
-        labels = inputs.get("labels")
+        labels = inputs.pop("labels")
         # forward pass
         outputs = model(**inputs)
         logits = outputs.get("logits")