make schur factorization differentiable

[baggepinnen]

Waiting for a new release that incorporates

• support functions that return scalars JuliaDecisionFocusedLearning/ImplicitDifferentiation.jl#51

[codecov]

Codecov Report

Merging #849 (39a90d4) into master (42e3955) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #849   +/-   ##
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  Coverage   92.72%   92.72%           
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  Files           5        5           
  Lines         330      330           
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  Hits          306      306           
  Misses         24       24           

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[gdalle]

Can you merge this without that release of ImplicitDifferentiation? I'm not yet sure how to make it work in reverse mode