documentation: gpu pypi url

[nim65s]

README + https://agimus-project.github.io/happypose/ are showing

pip install .[pypi,cpu] --extra-index-url https://download.pytorch.org/whl/cpu

we have to explain that for gpu, there is another index url.

thanks @Kotochleb for raising this

[nim65s]

hum, actually, url for GPU depends on cuda version. And it might simply be pypi, as it is currently in pyproject.toml.

Let's wait for some pytorch / cuda / nvidia drivers updates, tests and decisions.