def get_sequence_indices(self) -> List[List[int]]:
        """Build sequence indices.

        The default method creates sample indices that each sample is a single
        frame (i.e. seq_len=1). Override this method in the subclass to define
        how frames are sampled to form data samples.

        Outputs:
            sample_indices: the frame indices of each sample.
                For a sample, all frames will be treated as an input sequence,
                and the ground-truth pose of the last frame will be the target.
        """
        sequence_indices = []
        if self.seq_len == 1:
            num_imgs = len(self.ann_data['imgname'])
            sequence_indices = [[idx] for idx in range(num_imgs)]
        else:
            raise NotImplementedError('Multi-frame data sample unsupported!')
        return sequence_indices
Build sequence indices.

        The default method creates sample indices that each sample is a single
        frame (i.e. seq_len=1). Override this method in the subclass to define
        how frames are sampled to form data samples.

        Outputs:
            sample_indices: the frame indices of each sample.
                For a sample, all frames will be treated as an input sequence,
                and the ground-truth pose of the last frame will be the target.
        
mmpose/mmpose/datasets/datasets/base/base_mocap_dataset.py