Consider isolating phate dependency more #263
Description
Phate depends on s-gd2, which does not distribute wheels for python>=3.12, and relies on a legacy build configuration (jxz12/s_gd2#34). This means that currently users would need to have a working C++ compiler when they install viscy with the metrics extra, which is required for training (including virtual staining).
We should consider isolating this package in its own dependency group to avoid installation failure (e.g. installing on Windows requires a Visual Studio setup that is not obvious to python developers). Also imports from phate need to be audited so that they don't prevent other features from working if it's not installed.