Save state also for backward pass

src/pyronn_torch/conebeam.py

@@ -85,6 +85,7 @@ class _ForwardProjection(torch.autograd.Function):
             state.projection_multiplier, volume_grad, *state.volume_origin,
             *state.volume_spacing)
 
+        self.state = state
         if return_none:
             return volume_grad, None
         else:
@@ -92,8 +93,8 @@ class _ForwardProjection(torch.autograd.Function):
 
 
 class _BackwardProjection(torch.autograd.Function):
-    backward = _ForwardProjection.forward
-    forward = _ForwardProjection.backward
+    backward = staticmethod(_ForwardProjection.forward)
+    forward = staticmethod(_ForwardProjection.backward)
 
 
 class ConeBeamProjector: