Fix return type of piq.ssim

This function can return a single tensor if the parameter `full` is set to `False` (the default).

piq/ssim.py

@@ -19,7 +19,7 @@
 
 def ssim(x: torch.Tensor, y: torch.Tensor, kernel_size: int = 11, kernel_sigma: float = 1.5,
          data_range: Union[int, float] = 1., reduction: str = 'mean', full: bool = False,
-         downsample: bool = True, k1: float = 0.01, k2: float = 0.03) -> List[torch.Tensor]:
+         downsample: bool = True, k1: float = 0.01, k2: float = 0.03) -> List[torch.Tensor] | torch.Tensor:
     r"""Interface of Structural Similarity (SSIM) index.
     Inputs supposed to be in range ``[0, data_range]``.
     To match performance with skimage and tensorflow set ``'downsample' = True``.