def on_test_start(self, trainer: Trainer, pl_module: LightningModule) -> None:
        """Called when test begins.

        Records test start time and counts total number of test images.

        Args:
            trainer (Trainer): PyTorch Lightning trainer instance
            pl_module (LightningModule): The current training module

        Note:
            - Records start timestamp for testing phase
            - Counts total images across all test dataloaders if multiple are present
            - The module argument is not used but kept for callback signature compatibility
        """
        del pl_module  # Unused argument.

        self.start = time.time()
        self.num_images = 0

        if trainer.test_dataloaders is not None:  # Check to placate Mypy.
            if isinstance(trainer.test_dataloaders, torch.utils.data.dataloader.DataLoader):
                self.num_images += len(trainer.test_dataloaders.dataset)
            else:
                for dataloader in trainer.test_dataloaders:
                    self.num_images += len(dataloader.dataset)
Called when test begins.

        Records test start time and counts total number of test images.

        Args:
            trainer (Trainer): PyTorch Lightning trainer instance
            pl_module (LightningModule): The current training module

        Note:
            - Records start timestamp for testing phase
            - Counts total images across all test dataloaders if multiple are present
            - The module argument is not used but kept for callback signature compatibility
        
src/anomalib/callbacks/timer.py