def _join_root(self, path):
        """Join ``self.data_root`` with the specified path.

        If the path is an absolute path, just return the path. And if the
        path is None, return ``self.data_root``.

        Examples:
            >>> self.data_root = 'a/b/c'
            >>> self._join_root('d/e/')
            'a/b/c/d/e'
            >>> self._join_root('https://openmmlab.com')
            'https://openmmlab.com'
            >>> self._join_root(None)
            'a/b/c'
        """
        if path is None:
            return self.data_root
        if isabs(path):
            return path

        joined_path = self.file_backend.join_path(self.data_root, path)
        return joined_path
Join ``self.data_root`` with the specified path.

        If the path is an absolute path, just return the path. And if the
        path is None, return ``self.data_root``.

        Examples:
            >>> self.data_root = 'a/b/c'
            >>> self._join_root('d/e/')
            'a/b/c/d/e'
            >>> self._join_root('https://openmmlab.com')
            'https://openmmlab.com'
            >>> self._join_root(None)
            'a/b/c'
        
mmpretrain/datasets/multi_task.py