[`MBart`] Fix cross attention mask check (#21730)

fix typo

src/transformers/models/mbart/modeling_mbart.py

@@ -1055,7 +1055,7 @@ class MBartDecoder(MBartPreTrainedModel):
                 if attn_mask.size()[0] != len(self.layers):
                     raise ValueError(
                         f"The `{mask_name}` should be specified for {len(self.layers)} layers, but it is for"
-                        f" {head_mask.size()[0]}."
+                        f" {attn_mask.size()[0]}."
                     )
         for idx, decoder_layer in enumerate(self.layers):
             # add LayerDrop (see https://arxiv.org/abs/1909.11556 for description)