Miscellaneous Refactors

[luciaquirke]

• Always save the eigendecomposition of a matrix and always assume it is saved and loaded in that way. Comes in handy when you want to reuse precondiitoners, e.g. do trackstar once with and without unit_normalize
• In damped_psd_power we may want to remove the optional regularizer - not sure if anyone ever wants a regularizer that isn't scaled identity
• Add score path that is low-VRAM (doesn't copy index grads via stack or cat)
• Merge all the preconditioner functionalities (i.e. the EKFAC ones etc) into a general preconditioner processing function and update callsites (e.g. Attributor)
• Switch from print statements to logger