Make explicit that CUDA support is only available through PyTorch index URL

[scotts]

📚 The doc issue

We have some user confusion (#442) about which installs have CUDA support. We should make it more explicit in our README installation instructions that the pip install torchcodec recipe is strictly CPU, and that CUDA support has to come through the PyTorch index URLs.

[scotts]

I do note that PyTorch actually has CUDA 12.4 as the target for pip3 install torch torchvision torchaudio; see https://pytorch.org/get-started/locally/. We could do that in the future. That is, make what is on pypi the latest CUDA version. But, for this release, we definitely have this behavior so we should document it more clearly.