revert single cell grad approx CM

Author: tylerflex

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - Gradient inaccuracy when a multi-frequency monitor is used but a single frequency is selected.
+- Revert single cell center approximation for custom medium gradient.
 
 ## [2.7.7] - 2024-11-15
 

tidy3d/components/structure.py

@@ -260,20 +260,6 @@ def make_adjoint_monitors(
         if isinstance(self.geometry, PolySlab):
             size[self.geometry.axis] = 0
 
-        # custom medium only needs fields at center locations of unit cells.
-        if isinstance(self.medium, CustomMedium):
-            for axis, dim in enumerate("xyz"):
-                if self.medium.permittivity is not None:
-                    if len(self.medium.permittivity.coords[dim]) == 1:
-                        size[axis] = 0
-                if self.medium.eps_dataset is not None:
-                    zero_size = True
-                    for _, fld in self.medium.eps_dataset.field_components.items():
-                        if len(fld.coords[dim]) != 1:
-                            zero_size = False
-                    if zero_size:
-                        size[axis] = 0
-
         mnt_fld = FieldMonitor(
             size=size,
             center=center,