Fix error message on MS-SSIM

[JessyD]

Fix the new line on the error messages of the MS-SSIM metric are displayed. For example, this is how it currently looks like

--> 117         raise ValueError(
    118             f"Image size should be larger than {bigger_than} due to \
    119                 the {len(self.weights) - 1} downsamplings in MS-SSIM."
    120         )
    122 levels = self.weights.shape[0]
    123 mcs_list: List[torch.Tensor] = []

ValueError: Image size should be larger than 144 due to                         the 4 downsamplings in MS-SSIM.