Ekfac

[LouisYRYJ]

This introduces EKFAC computations, everything relevant is in /hessians

• ignore /approx_unrolling for this PR
• gradient_covariance files in hessians are for debugging, ignore these too
• I mostly followed the build_index pipeline
• Currently, we should not merge this, because it uses the current version main and not the ultrafast one
  ->As I refactored some parts of the general pipeline, we should probably merge ultrafast first and then I can make the changes to this PR.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ LouisYRYJ
❌ root

---

root seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}