install callback to executor.
        Supports installing to multiple exes.

        Parameters
        ----------
        exe : mx.executor.Executor
            The Executor (returned by symbol.bind) to install to.

def install(self, exe):
        """install callback to executor.
        Supports installing to multiple exes.

        Parameters
        ----------
        exe : mx.executor.Executor
            The Executor (returned by symbol.bind) to install to.
        """
        exe.set_monitor_callback(self.stat_helper, self.monitor_all)
        self.exes.append(exe)