Flax beam search fix (#21857)

src/transformers/generation/flax_utils.py

@@ -448,9 +448,10 @@ class FlaxGenerationMixin:
                     model_kwargs["encoder_outputs"]["last_hidden_state"], num_beams=generation_config.num_beams
                 )
 
-            if "attention_mask" in model_kwargs:
-                model_kwargs["attention_mask"] = self._expand_to_num_beams(
-                    model_kwargs["attention_mask"], num_beams=generation_config.num_beams
+            for kwarg in ["attention_mask", "decoder_attention_mask"]:
+                if kwarg in model_kwargs:
+                    model_kwargs[kwarg] = self._expand_to_num_beams(
+                        model_kwargs[kwarg], num_beams=generation_config.num_beams
                     )
 
             return self._beam_search(
@@ -821,8 +822,9 @@ class FlaxGenerationMixin:
             model_kwargs["encoder_outputs"]["last_hidden_state"] = flatten_beam_dim(
                 model_kwargs["encoder_outputs"]["last_hidden_state"]
             )
-        if "attention_mask" in model_kwargs:
-            model_kwargs["attention_mask"] = flatten_beam_dim(model_kwargs["attention_mask"])
+        for kwarg in ["attention_mask", "decoder_attention_mask"]:
+            if kwarg in model_kwargs:
+                model_kwargs[kwarg] = flatten_beam_dim(model_kwargs[kwarg])
 
         # initialize model specific kwargs
         model_kwargs = self.prepare_inputs_for_generation(flatten_beam_dim(input_ids), max_length, **model_kwargs)