NL is number of scale levels of the scale space, see SIFT paper, Figure 1 on the left. This mean to consequently blur the image more and more, which for linear filters is equivalent to resizing.
If you just need a pyramid, which contains HW, H/2 W/2, H/4 W/4 and so on - use kornia.geometry.transform — Kornia documentation 5 function instead. It does not have levels unlike scale-space pyramid, that is a mistake in documentation (fixed in master)

  
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lowe-ijcv04.pdf 1

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