Fix `test_finetune_bert2bert` (#25984)

Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

src/transformers/trainer_seq2seq.py

@@ -281,15 +281,16 @@ class Seq2SeqTrainer(Trainer):
             gen_kwargs["synced_gpus"] if gen_kwargs.get("synced_gpus") is not None else default_synced_gpus
         )
 
+        generation_inputs = inputs.copy()
         # If the `decoder_input_ids` was created from `labels`, evict the former, so that the model can freely generate
         # (otherwise, it would continue generating from the padded `decoder_input_ids`)
         if (
-            "labels" in inputs
-            and "decoder_input_ids" in inputs
-            and inputs["labels"].shape == inputs["decoder_input_ids"].shape
+            "labels" in generation_inputs
+            and "decoder_input_ids" in generation_inputs
+            and generation_inputs["labels"].shape == generation_inputs["decoder_input_ids"].shape
         ):
-            inputs = {k: v for k, v in inputs.items() if k != "decoder_input_ids"}
-        generated_tokens = self.model.generate(**inputs, **gen_kwargs)
+            generation_inputs = {k: v for k, v in inputs.items() if k != "decoder_input_ids"}
+        generated_tokens = self.model.generate(**generation_inputs, **gen_kwargs)
 
         # Temporary hack to ensure the generation config is not initialized for each iteration of the evaluation loop
         # TODO: remove this hack when the legacy code that initializes generation_config from a model config is