How to use the delve.writers.NPYWriter function in delve

def _warn_if_covariance_not_saveable(self, stats: List[str]):
        warn = False
        if 'cov' in stats:
            if isinstance(self.writer, CompositWriter):
                for writer in self.writer.writers:
                    if isinstance(writer, NPYWriter):
                        return
                warn = True
            elif not isinstance(self.writer, NPYWriter):
                warn = True
        if warn:
            warnings.warn("'cov' was selected as stat, but 'npy' (NPYWriter)"
                          "is not used as a save strategy, which is the only"

def _warn_if_covariance_not_saveable(self, stats: List[str]):
        warn = False
        if 'cov' in stats:
            if isinstance(self.writer, CompositWriter):
                for writer in self.writer.writers:
                    if isinstance(writer, NPYWriter):
                        return
                warn = True
            elif not isinstance(self.writer, NPYWriter):
                warn = True
        if warn:
            warnings.warn("'cov' was selected as stat, but 'npy' (NPYWriter)"
                          "is not used as a save strategy, which is the only"

def __init__(self, savepath: str, zip: bool = False, **kwargs):
        """
        The NPYWriter creates a folder containing one subfolder for each stat.
        Each subfolder contains a npy-file with the saturation value for each epoch.
        This writer saves non-scalar values and can thus be used to save
        the covariance-matrix.
        :param savepath: The root folder to save the folder structure to
        :param zip: Whether to zip the output folder after every invocation
        """
        super(NPYWriter, self).__init__()
        self.savepath = savepath
        self.epoch_counter = {}
        self.zip = zip