Fix Seq2seqTrainer decoder attention mask (huggingface#26841)

Don't drop decoder_input_ids without also dropping decoder_attention_mask

src/transformers/trainer_seq2seq.py

@@ -288,7 +288,9 @@ def prediction_step(
             and "decoder_input_ids" in generation_inputs
             and generation_inputs["labels"].shape == generation_inputs["decoder_input_ids"].shape
         ):
-            generation_inputs = {k: v for k, v in inputs.items() if k != "decoder_input_ids"}
+            generation_inputs = {
+                k: v for k, v in inputs.items() if k not in ("decoder_input_ids", "decoder_attention_mask")
+            }
         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