Remove torchvision from the PyTorch install instructions

Change: we are removing torchvision from the install commands on pytorch.org/get-started, which will install torch only. Applied to nightly now; it will be removed from the stable instructions starting with the 2.16 release.

# before
pip3 install --pre torch torchvision --index-url https://download.pytorch.org/whl/nightly/cu132
# starting today
pip3 install --pre torch --index-url https://download.pytorch.org/whl/nightly/cu132

TorchVision is ABI stable now (0.29 release notes): it is ABI stable w.r.t. torch 2.14, so torchvision 0.29 is compatible with future torch versions (2.15, 2.16, etc.). You don’t need to install a new TorchVision when you upgrade torch. As a result, TorchVision may stop releasing in sync with PyTorch. It is still actively maintained and developed, with releases at a different cadence. Pinning the two together in one command no longer matches how they ship.