fix: Ensure 'saves' directory exists for output files in example scripts

[wanhhe]

Create 'saves' directory if it does not exist before saving audio when running example infer_s2s.py

Problem:
Since torchaudio.save() does not automatically create directories, running the example script examples/infer_s2s.py directly will result in an error.

  File "/root/miniconda3/lib/python3.12/site-packages/soundfile.py", line 343, in write
    with SoundFile(file, 'w', samplerate, channels,
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/miniconda3/lib/python3.12/site-packages/soundfile.py", line 658, in __init__
    self._file = self._open(file, mode_int, closefd)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/miniconda3/lib/python3.12/site-packages/soundfile.py", line 1216, in _open
    raise LibsndfileError(err, prefix="Error opening {0!r}: ".format(self.name))
soundfile.LibsndfileError: Error opening 'saves/output_audio_xxx.wav': System error.