Thank you. Effectively, I downgraded to the Linux Kernel 5.8.5 from Arch Linux Archive, I removed nvidia, then I first installed linux-headers-5.8.5, then nvidia-dkms; it is worth noting that linux-headers-5.8.5 must imperatively be installed before nvidia-dkms, otherwise the kernel headers of the Linux kernel 5.8.5 will be missing for nvidia-dkms.
Now it works and PyTorch detects CUDA.