Use dy=true for gradient (smallest possible 1)

[gdalle]

DI source

• Replace dy = one(eltype(x)) (which doesn't work for non-arrays) with dy = true for the pullback that defines gradient
• Modify a few type annotations

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.57%. Comparing base (63fa856) to head (1bc0dde).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #363      +/-   ##
==========================================
- Coverage   96.57%   96.57%   -0.01%     
==========================================
  Files          98       98              
  Lines        4813     4811       -2     
==========================================
- Hits         4648     4646       -2     
  Misses        165      165              

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