[NFC] Export `jacobian_default_mode`
Created by: PhilipVinc
Move and renames the internal chose_qgt
from the qgt module to nk.jax.jacobian_default_mode
, and properly documents it.
It turns out it's useful to implement NTK and other objects, and GPSKet uses it as well, so let's make it public API together with Jacobian..
This changes nothing, simply moves a function (adds a deprecation warning for the internal function in case anybody uses this function) and adds more extensive docstrings.