def METHOD_NAME(self, predictions, batch, stage=sb.Stage.TRAIN):
    """
    this function computes the l2-error to train the NMF model.
    """
    batch = batch.to(self.device)
    wavs, lens = batch.sig
    X_stft = self.hparams.compute_stft(wavs)
    X_stft_power = self.hparams.compute_stft_mag(X_stft)
    target = torch.log1p(X_stft_power).permute(0, 2, 1)
    loss = ((target.squeeze() - predictions) ** 2).mean()
    return loss