Make the LineSearch gradient parameter in BFGS consistent with the impl

[dspyz-matician]

I want to implement my own custom Param type, but use Array1<f32> as the gradient type. However, the current constraints essentially force these types to be the same because the BacktrackingLineSearch trait is only implemented for <G, F> not for <P, F>

This PR fixes BFGS's constraints so that G is used consistently as the parameter to BFGS's associated LineSearch

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.48%. Comparing base (a9e3eb7) to head (15d60af).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #508      +/-   ##
==========================================
- Coverage   92.08%   91.48%   -0.61%     
==========================================
  Files         178      178              
  Lines       24455    23623     -832     
==========================================
- Hits        22520    21612     -908     
- Misses       1935     2011      +76     

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[stefan-k]

Thank you! :)