Your Ampere GPU (RTX 3090) needs CUDA>=11.0 and I don’t know which exact PyTorch binary (or source build) you’ve installed.
In any case, the CUDA11 support came on PyTorch ~1.7 so your 1.5.0 installation will most likely be using CUDA10, which is causing the error.



 giampierosalvi:

Both machines have cuda V10.1.243.


The pip wheels and conda binaries ship with their own CUDA runtime and your local CUDA toolkit is used for a source build or to build custom CUDA extensions.