typo (#6959)

there is no var `decoder_input_ids`, but there is `input_ids` for decoder :)

src/transformers/generation_utils.py

@@ -411,7 +411,7 @@ class GenerationMixin:
             )  # shape: (batch_size * num_return_sequences * num_beams, cur_len)
 
         if self.config.is_encoder_decoder:
-            # create empty decoder_input_ids
+            # create empty decoder input_ids
             input_ids = torch.full(
                 (effective_batch_size * num_beams, 1),
                 decoder_start_token_id,