change normals grad calculation (#1108)

* change normals grad calculation

* changing normal calc

nerfstudio/fields/base_field.py

@@ -74,10 +74,15 @@ class Field(nn.Module):
             self._sample_locations.shape[:-1] == self._density_before_activation.shape[:-1]
         ), "Sample locations and density must have the same shape besides the last dimension."
 
-        self._density_before_activation.backward(
-            gradient=torch.ones_like(self._density_before_activation), inputs=self._sample_locations, retain_graph=True
-        )
-        normals = -torch.nn.functional.normalize(self._sample_locations.grad, dim=-1)
+        normals = torch.autograd.grad(
+            self._density_before_activation,
+            self._sample_locations,
+            grad_outputs=torch.ones_like(self._density_before_activation),
+            retain_graph=True,
+        )[0]
+
+        normals = -torch.nn.functional.normalize(normals, dim=-1)
+
         return normals
 
     @abstractmethod