add missing mido import to main.py

main.py

@@ -4,6 +4,7 @@ from fastapi.responses import FileResponse
 from pydantic import BaseModel
 from pydub import AudioSegment
 from pydub.effects import normalize, compress_dynamic_range
+import mido
 from mido import MidiFile, MidiTrack, Message
 import copy
 import torch