import torch


def test_mask() -> torch.Tensor:
    """Single 4x4 test mask with index labels.

    :return: 4x4 single-channel index tensor
    :rtype: torch.Tensor
    """
    return torch.tensor([[0, 1, 3, 3], [2, 4, 2, 1], [1, 2, 2, 0], [2, 255, 255, 255]], dtype=torch.int)