    def _copy_keys(args, cls, prefix, seen):
        """
        copy the prefixed keys (decoder_embed_dim) to the DC fields: decoder.embed_dim
        """
        cfg = cls()
        for fld in fields(cls):
            # for all the fields in the DC, find the fields (e.g. embed_dim)
            # in the namespace with the prefix (e.g. decoder)
            # and set it on the dc.
            args_key = f"{prefix}_{fld.name}"
            if safe_hasattr(args, args_key):
                seen.add(args_key)
                setattr(cfg, fld.name, safe_getattr(args, args_key))
            if safe_hasattr(args, fld.name):
                seen.add(fld.name)
                setattr(cfg, fld.name, safe_getattr(args, fld.name))
        return cfg
copy the prefixed keys (decoder_embed_dim) to the DC fields: decoder.embed_dim
fairseq/models/transformer/transformer_config.py