Apply suggestion from @ericspod

Signed-off-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>

Author: ericspod

monai/losses/image_dissimilarity.py

@@ -321,7 +321,8 @@ def parzen_windowing_gaussian(self, img: torch.Tensor) -> tuple[torch.Tensor, to
         img = torch.clamp(img, 0, 1)
         img = img.reshape(img.shape[0], -1, 1)  # (batch, num_sample, 1)
         if self.bin_centers is None:
-            raise ValueError("bin_centers must be defined for gaussian parzen windowing.")
+        if self.bin_centers is None or self.preterm is None:
+            raise ValueError("bin_centers and preterm must be defined for gaussian parzen windowing.")
         weight = torch.exp(
             -self.preterm.to(img) * (img - self.bin_centers.to(img)) ** 2
         )  # (batch, num_sample, num_bin)