Return symbol for target backend.

        Parameters
        ----------
        backend : str
            The backend names.

        Returns
        -------
        out : Symbol
            The created Symbol for target backend.

def get_backend_symbol(self, backend):
        """Return symbol for target backend.

        Parameters
        ----------
        backend : str
            The backend names.

        Returns
        -------
        out : Symbol
            The created Symbol for target backend.
        """
        out = SymbolHandle()
        check_call(_LIB.MXGenBackendSubgraph(self.handle, c_str(backend), ctypes.byref(out)))
        return Symbol(out)