I have figured this out. That’s because Tensor ‘x0_0’ is not only concatenated with ‘up’, but also is passed to ‘x1_0’. So the final grads of  ‘x0_0’ come from two parts. And the two parts grads are added directly.
Thank for your reply. And I’d like to ask where can I take a look the backward formulas of different PyTorch operations? I want to know how grads are computed for some specified functions, like F.interpolate()?