I found that /usr/share/nvidia/nvoptix.bin was missing. I downloaded the driver from https://www.nvidia.com/en-us/drivers/unix/linux-amd64-display-archive/ and copied the binary file to its position.
sh NVIDIA-Linux-x86_64-535.54.03.run --extract-only
cd NVIDIA-Linux-x86_64-535.54.03
cp nvoptix.bin /usr/share/nvidia/And python -c 'import mitsuba; mitsuba.set_variant("cuda_ad_rgb"); denoiser = mitsuba.OptixDenoiser((640, 320))' runs without any error.