Hi @leeseng
There is currently a known CDN outage for the developer.download.nvidia.com domain with files showing as unavailable.
On a separate note, the NVIDIA Machine Learning repository is deprecated with cuDNN, NCCL, and TensorRT products now available in the CUDA repository.
For now you can remove the appropriate .list file in /etc/apt/sources.list.d/.
In the future, please report similar issues on GitHub: https://github.com/NVIDIA/cuda-repo-management/issues/1