I download the example file here:
https://www.sidefx.com/docs/houdini//nodes/lop/labs--karma_aovs_for_prman_denoiser-1.0.html [www.sidefx.com]

and the renderman denoiser tool give an error:

RuntimeError: There is no default part for layer 'RGB'

I also checked the debug info
channels: ['R', 'G', 'B', 'A', 'Ci.b', 'Ci.g', 'Ci.r'...

seems the denoiser want a seprated rgba channels