Hotfix for failing `MusicgenForConditionalGeneration` tests (#25091)

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

src/transformers/generation/utils.py

@@ -1154,17 +1154,22 @@ class GenerationMixin:
 
             # allow encoder kwargs
             encoder = getattr(self, "encoder", None)
-            if encoder is None:
-                encoder = getattr(base_model, "encoder")
+            # `MusicgenForConditionalGeneration` has `text_encoder` and `audio_encoder`.
+            # Also, it has `base_model_prefix = "encoder_decoder"` but there is no `self.encoder_decoder`
+            # TODO: A better way to handle this.
+            if encoder is None and base_model is not None:
+                encoder = getattr(base_model, "encoder", None)
 
+            if encoder is not None:
                 encoder_model_args = set(inspect.signature(encoder.forward).parameters)
                 model_args |= encoder_model_args
 
             # allow decoder kwargs
             decoder = getattr(self, "decoder", None)
-            if decoder is None:
-                decoder = getattr(base_model, "decoder")
+            if decoder is None and base_model is not None:
+                decoder = getattr(base_model, "decoder", None)
 
+            if decoder is not None:
                 decoder_model_args = set(inspect.signature(decoder.forward).parameters)
                 model_args |= {f"decoder_{x}" for x in decoder_model_args}