How to use the spleeter.audio.convertor.to_stereo function in spleeter

def separate(self, waveform):
        """ Performs source separation over the given waveform.

        The separation is performed synchronously but the result
        processing is done asynchronously, allowing for instance
        to export audio in parallel (through multiprocessing).

        Given result is passed by to the given consumer, which will
        be waited for task finishing if synchronous flag is True.

        :param waveform: Waveform to apply separation on.
        :returns: Separated waveforms.
        """
        if not waveform.shape[-1] == 2:
            waveform = to_stereo(waveform)
        predictor = self._get_predictor()
        prediction = predictor({
            'waveform': waveform,
            'audio_id': ''})
        prediction.pop('audio_id')
        return prediction