Add `prior_structure` argument to `optimize_prior_precision`

[runame]

Changes:

• Add a helper function fix_prior_prec_structure for marglik optimization.
• Add prior_structure argument to optimize_prior_precision, such that we can easily optimize a layer-wise or diagonal prior precision post-hoc.
• Rename the method CV for optimize_prior_precision to gridsearch.

[aleximmer]

Should we set the grid search as default and/or put a warning for post hoc prior precision optimization since the method has not been thoroughly evaluated, yet, and can lead to issues apparently?

[runame]

Should we set the grid search as default and/or put a warning for post hoc prior precision optimization since the method has not been thoroughly evaluated, yet, and can lead to issues apparently?

Yeah, maybe setting gridsearch as the default makes sense. A warning is a bit annoying since we cannot easily detect if someone is using optimize_prior_precision for marglik optimization post-hoc or online.

[wiseodd]

Rename the method CV' foroptimize_prior_precision' to `gridsearch'.

A possible downside is that people have accustomed to CV. Since optimize_prior_precision is a public-facing method and is almost always called by users, I think it's a bit dangerous to rename it. At the very least we should support both.