[WIP] Upgrade qml.structure_constants to handle dense matrices

[Qottmann]

Integrate structure constants dense into [sc-81966]

Todo

• update and unify documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.59%. Comparing base (2d784a3) to head (d7c93fa).

Additional details and impacted files

@@                  Coverage Diff                   @@
##           dla_lie_closure_dense    #6861   +/-   ##
======================================================
  Coverage                  99.59%   99.59%           
======================================================
  Files                        479      479           
  Lines                      45369    45398   +29     
======================================================
+ Hits                       45187    45216   +29     
  Misses                       182      182           

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[Qottmann]

Noticed that qml.math.diagonal dispatches to tf.linalg.diag_part instead of tf.keras.ops.diagonal , which to me seems like the natural fit when looking at other interfaces:

numpy.diagonal
torch.diagonal

I changed this, but might open pandora's box in the test suite 😬