[NFC] Export jacobian_default_mode

[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.

[PhilipVinc]

cc @yannra @maxbortone I see you're using it.
I'm moving it to somewhere more relevant, and were it can be kept stable

[codecov]

Codecov Report

Merging #1473 (b78e9fe) into master (b970084) will decrease coverage by 1.23%.
The diff coverage is 86.27%.

@@            Coverage Diff             @@
##           master    #1473      +/-   ##
==========================================
- Coverage   83.74%   82.52%   -1.23%     
==========================================
  Files         238      239       +1     
  Lines       13667    13456     -211     
  Branches     2075     2075              
==========================================
- Hits        11445    11104     -341     
- Misses       1708     1821     +113     
- Partials      514      531      +17     

Impacted Files	Coverage Δ
netket/optimizer/qgt/qgt_jacobian_common.py	94.44% <50.00%> (+1.38%)	⬆️
netket/jax/_jacobian/default_mode.py	88.37% <88.37%> (ø)
netket/jax/__init__.py	100.00% <100.00%> (ø)
netket/jax/_jacobian/__init__.py	100.00% <100.00%> (ø)
netket/optimizer/qgt/qgt_jacobian_dense.py	97.91% <100.00%> (ø)
netket/optimizer/qgt/qgt_jacobian_pytree.py	99.03% <100.00%> (ø)

... and 102 files with indirect coverage changes