def _test(n_epochs, metric_device):
        metric_device = torch.device(metric_device)
        n_iters = 80
        s = 16
        n_classes = 2

        offset = n_iters * s
        y_true = torch.rand(size=(offset * idist.get_world_size(),)).to(device)
        y_preds = torch.rand(size=(offset * idist.get_world_size(),)).to(device)

        def update(engine, i):
            return (
                y_preds[i * s + rank * offset : (i + 1) * s + rank * offset],
                y_true[i * s + rank * offset : (i + 1) * s + rank * offset],
            )

        engine = Engine(update)

        m = WaveHedgesDistance(device=metric_device)
        m.attach(engine, "whm")

        data = list(range(n_iters))
        engine.run(data=data, max_epochs=n_epochs)

        assert "whm" in engine.state.metrics

        res = engine.state.metrics["whm"]

        np_y_true = y_true.cpu().numpy()
        np_y_preds = y_preds.cpu().numpy()

        np_sum = (np.abs(np_y_true - np_y_preds) / (np.maximum.reduce([np_y_preds, np_y_true]) + 1e-30)).sum()

        assert pytest.approx(res) == np_sum
    def _test(y_pred, y, batch_size):
        def update_fn(engine, batch):
            idx = (engine.state.iteration - 1) * batch_size
            y_true_batch = np_y[idx : idx + batch_size]
            y_pred_batch = np_y_pred[idx : idx + batch_size]
            return torch.from_numpy(y_pred_batch), torch.from_numpy(y_true_batch)

        engine = Engine(update_fn)

        m = WaveHedgesDistance()
        m.attach(engine, "whd")

        np_y = y.numpy().ravel()
        np_y_pred = y_pred.numpy().ravel()

        data = list(range(y_pred.shape[0] // batch_size))
        whd = engine.run(data, max_epochs=1).metrics["whd"]

        np_sum = (np.abs(np_y - np_y_pred) / np.maximum.reduce([np_y_pred, np_y])).sum()

        assert np_sum == pytest.approx(whd)