Hi! Well it almost worked… and thank you very much. I say almost because I think the colors are inverted.
With torch.clamp for output tensor and np.clip for my FFT convolution, the inversion (observe the back of the head of the bird) is much more prominent:

sample_output_21536×761 361 KB

The edges are very similar but its the color I cannot understand why, are different. I will keep digging but do let me know if you have any clues.
Edit: it seems like either me or torch flips the kernel and then applies convolution. I flipped the kernel before converting it into weights and now I have the perfect replica. A few details remain, yet to be discovered!