The binaries for the current PyTorch release 1.8.1 and the nightly ship with CUDA10.2 and CUDA11.1 as given in the install instructions 76.
Your local CUDA9.1 installation won’t be used, if you are installing the conda binaries or pip wheels. Also note that you would need a newer NVIDIA driver, since even CUDA9.1 needs >=390.46 based on Table 1 24.