Optimal sparse decompression

[gdalle]

Core

• Make sparsity column-major for column-wise Jacobians (forward mode) and Hessians
• Make sparsity row-major for row-wise Jacobians (reverse mode)
• Generate the Jacobian / Hessian according to the same pattern, respecting major
• Implement decompress! for SparseMatrixCSC (when dir = :col) and its transpose (when dir = :row)
• Give up on symmetric coloring and decompressing for Hessians at the moment

Tests

• Quit testing typeof(jac) == SparseMatrixCSC because we can have a Transpose of that (and more general stuff in the future)
• Test sparsity with AutoSparse(AutoEnzyme(Enzyme.Reverse)) to cover the reverse mode mutating Jacobian

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.17%. Comparing base (09a7da0) to head (2e04819).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #223      +/-   ##
==========================================
+ Coverage   95.64%   96.17%   +0.52%     
==========================================
  Files          74       74              
  Lines        3839     3846       +7     
==========================================
+ Hits         3672     3699      +27     
+ Misses        167      147      -20     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.