fix polyslab edge gradient assignment

Author: tylerflex

CHANGELOG.md

@@ -24,7 +24,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Bug when differentiating with respect to `Cylinder.center`.
 - `xarray` 2024.10.0 compatibility for autograd.
 - Some failing examples in the expressions plugin documentation.
-
+- Inaccuracy in transforming gradients from edge to `PolySlab.vertices`.
 
 ## [2.7.6] - 2024-10-30
 

tidy3d/components/geometry/polyslab.py

@@ -1448,7 +1448,7 @@ def compute_derivative_vertices(self, derivative_info: DerivativeInfo) -> Traced
 
         vjps_edges_in_plane = vjps_edges.values.reshape((num_vertices, 1)) * normal_vectors_in_plane
 
-        vjps_vertices = vjps_edges_in_plane + np.roll(vjps_edges_in_plane, axis=0, shift=-1)
+        vjps_vertices = vjps_edges_in_plane + np.roll(vjps_edges_in_plane, axis=0, shift=1)
         vjps_vertices /= 2.0  # each vertex is effected only 1/2 by each edge
 
         # sign change if counter clockwise, because normal direction is flipped