    def __getitem__(self, index):
        """
        single_feature: dict,
            wav: (T,)
            wav_len: int
            target_len: int
            mask: (n_frames, 1)
            spk_id

            wav_{sr}: (T,)
            wav_{sr}_len: int
        """
        single_feature = dict()

        utt_item = self.metadata[index]
        wav_path = utt_item["Path"]

        ### Use the highest sampling rate to load and randomly select ###
        highest_sr_wav, _ = librosa.load(wav_path, sr=self.highest_sample_rate)
        highest_sr_wav = self.random_select(
            highest_sr_wav, utt_item["Duration"], wav_path
        )

        ### Waveforms under all the sample rates ###
        for sr in self.all_sample_rates:
            # Resample to the required sample rate
            if sr != self.highest_sample_rate:
                wav_sr = librosa.resample(
                    highest_sr_wav, orig_sr=self.highest_sample_rate, target_sr=sr
                )
            else:
                wav_sr = highest_sr_wav

            wav_sr = torch.as_tensor(wav_sr, dtype=torch.float32)
            single_feature["wav_{}".format(sr)] = wav_sr
            single_feature["wav_{}_len".format(sr)] = len(wav_sr)

            # For target sample rate
            if sr == self.sample_rate:
                wav_len = len(wav_sr)
                frame_len = wav_len // self.hop_size

                single_feature["wav"] = wav_sr
                single_feature["wav_len"] = wav_len
                single_feature["target_len"] = frame_len
                single_feature["mask"] = torch.ones(frame_len, 1, dtype=torch.long)

        ### Speaker ID ###
        if self.cfg.preprocess.use_spkid:
            utt = "{}_{}".format(utt_item["Dataset"], utt_item["Uid"])
            single_feature["spk_id"] = torch.tensor(
                [self.spk2id[self.utt2spk[utt]]], dtype=torch.int32
            )

        return single_feature
single_feature: dict,
wav: (T,)
wav_len: int
target_len: int
mask: (n_frames, 1)
spk_id

wav_{sr}: (T,)
wav_{sr}_len: int
models/svc/base/svc_dataset.py