Fix incorrect truncated parameter in make_gaussian_kernel causing corrupted LocalNormalizedCrossCorrelationLoss (#8783)

Fixes #8780 

### Description

Divide the pixel radius by `sigma` to convert it into the correct
sigma-unit ratio.

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

---------

Signed-off-by: ytl0623 <david89062388@gmail.com>
Signed-off-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>

Author: 3 people

tests/integration/test_reg_loss_integration.py

@@ -26,6 +26,7 @@
     [LocalNormalizedCrossCorrelationLoss, {"kernel_size": 7, "kernel_type": "rectangular"}, ["pred", "target"]],
     [LocalNormalizedCrossCorrelationLoss, {"kernel_size": 5, "kernel_type": "triangular"}, ["pred", "target"]],
     [LocalNormalizedCrossCorrelationLoss, {"kernel_size": 3, "kernel_type": "gaussian"}, ["pred", "target"]],
+    [LocalNormalizedCrossCorrelationLoss, {"kernel_size": 7, "kernel_type": "gaussian"}, ["pred", "target"]],
     [GlobalMutualInformationLoss, {"num_bins": 10}, ["pred", "target"]],
     [GlobalMutualInformationLoss, {"kernel_type": "b-spline", "num_bins": 10}, ["pred", "target"]],
 ]
@@ -98,6 +99,24 @@ def forward(self, x):
             optimizer.step()
         self.assertGreater(init_loss, loss_val, "loss did not decrease")
 
+    def test_lncc_gaussian_kernel_gt3_identical_images(self):
+        """
+        Regression test for make_gaussian_kernel truncated parameter bug.
+        LNCC on identical inputs must be close to -1.0 for gaussian kernel_size > 3.
+        """
+        for kernel_size in [5, 7]:
+            with self.subTest(kernel_size=kernel_size):
+                loss_fn = LocalNormalizedCrossCorrelationLoss(
+                    spatial_dims=2, kernel_size=kernel_size, kernel_type="gaussian"
+                ).to(self.device)
+                x = torch.rand(2, 1, 32, 32, device=self.device)
+                y = x.clone()
+                loss = loss_fn(x, y)
+                self.assertTrue(
+                    torch.allclose(loss, torch.tensor(-1.0, device=self.device, dtype=loss.dtype), atol=1e-3),
+                    f"LNCC of identical images should be -1.0, got {loss.item():.6f} (kernel_size={kernel_size})",
+                )
+
 
 if __name__ == "__main__":
     unittest.main()