mark idx in BallQuery as non_differentiable (#919)

pcdet/ops/pointnet2/pointnet2_stack/pointnet2_utils.py

@@ -35,11 +35,15 @@ class BallQuery(Function):
         pointnet2.ball_query_wrapper(B, M, radius, nsample, new_xyz, new_xyz_batch_cnt, xyz, xyz_batch_cnt, idx)
         empty_ball_mask = (idx[:, 0] == -1)
         idx[empty_ball_mask] = 0
+
+        ctx.mark_non_differentiable(idx)
+        ctx.mark_non_differentiable(empty_ball_mask)
+
         return idx, empty_ball_mask
 
     @staticmethod
-    def backward(ctx, a=None):
-        return None, None, None, None
+    def backward(ctx, a=None, b=None):
+        return None, None, None, None, None, None
 
 
 ball_query = BallQuery.apply