Fix UnboundLocalError for torchaudio in ReferenceLoader.__init__ (#1257)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

fish_speech/inference_engine/reference_loader.py

@@ -45,7 +45,7 @@ class ReferenceLoader:
             # torchaudio 2.9+ removed list_audio_backends()
             # Try ffmpeg first, fallback to soundfile
             try:
-                import torchaudio.io._load_audio_fileobj  # noqa: F401
+                __import__("torchaudio.io._load_audio_fileobj")
 
                 self.backend = "ffmpeg"
             except (ImportError, ModuleNotFoundError):