optimize dataset loading

class CCIPImagesDataset(ImagesDataset):

    def __init__(self, root_dir, transform=None):

        _ids, _maxid = {}, 0

        _maxid = 0

        _items: List[Tuple[str, int]] = []

        for file in glob.glob(os.path.join(root_dir, '*', '*', '*.jpg')):

            dirname = os.path.normcase(os.path.normpath(os.path.dirname(os.path.abspath(file))))

            if dirname not in _ids:

                _ids[dirname] = _maxid

                _maxid += 1

            _items.append((file, _ids[dirname]))

        for root, dirs, files in os.walk(root_dir, topdown=False):

            jpg_files = list(filter(lambda x:x.endswith('.jpg'), files))

            if len(jpg_files)>0:

                for name in jpg_files:

                    _items.append((os.path.join(root, name), _maxid))

                _maxid += 1

        ImagesDataset.__init__(self, _items, transform)

        super(CCIPImagesDataset, self).__init__(self, _items, transform)

class CharacterDataset(Dataset):