Remove fallback backend -> SecondOrder(backend, backend)

[gdalle]

DI source

• Use maybe_inner and maybe_outer to make the code for hvp and second_derivative work with standard backends too
• Remove unnecessary fallback from backend to SecondOrder(backend, backend)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.51%. Comparing base (1155218) to head (b83482a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #459      +/-   ##
==========================================
- Coverage   98.52%   98.51%   -0.01%     
==========================================
  Files         106      106              
  Lines        4265     4250      -15     
==========================================
- Hits         4202     4187      -15     
  Misses         63       63              

Flag	Coverage Δ
DI	98.66% <100.00%> (-0.01%)	⬇️
DIT	98.21% <ø> (ø)

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