Returns a string describing the total resources available.

def resource_string(self):
        """Returns a string describing the total resources available."""

        if self._resources_initialized:
            res_str = "{} CPUs, {} GPUs".format(self._avail_resources.cpu,
                                                self._avail_resources.gpu)
            if self._avail_resources.custom_resources:
                custom = ", ".join(
                    "{} {}".format(
                        self._avail_resources.get_res_total(name), name)
                    for name in self._avail_resources.custom_resources)
                res_str += " ({})".format(custom)
            return res_str
        else:
            return "? CPUs, ? GPUs"