Initialize a DocStructure object from a json dictionary.

def _from_dict(cls, _dict):
        """Initialize a DocStructure object from a json dictionary."""
        args = {}
        if 'section_titles' in _dict:
            args['section_titles'] = [
                SectionTitles._from_dict(x)
                for x in (_dict.get('section_titles'))
            ]
        if 'leading_sentences' in _dict:
            args['leading_sentences'] = [
                LeadingSentence._from_dict(x)
                for x in (_dict.get('leading_sentences'))
            ]
        return cls(**args)