fix: handle constant derivatives with runtime activity for Enzyme

[gdalle]

Fixes #666

Unfortunately, since it relies on pointer(y) == pointer(dy), it only works for Array at the moment. Any clue on how to generalize to arbitrary structs is welcome (keeping in mind that y === dy doesn't do what we want)

cc @wsmoses

[codecov]

Codecov Report

❌ Patch coverage is 0% with 51 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.12%. Comparing base (a5fb081) to head (02ae6b5).

Files with missing lines	Patch %	Lines
...ifferentiationInterfaceEnzymeExt/forward_onearg.jl	0.00%	18 Missing ⚠️
...fferentiationInterfaceTest/src/scenarios/modify.jl	0.00%	18 Missing ⚠️
...ace/ext/DifferentiationInterfaceEnzymeExt/utils.jl	0.00%	13 Missing ⚠️
...ifferentiationInterfaceEnzymeExt/reverse_onearg.jl	0.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (a5fb081) and HEAD (02ae6b5). Click for more details.

HEAD has 47 uploads less than BASE

Flag	BASE (a5fb081)	HEAD (02ae6b5)
DIT	12	2
DI	52	15

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #850       +/-   ##
===========================================
- Coverage   98.12%   79.12%   -19.00%     
===========================================
  Files         134      130        -4     
  Lines        7988     7972       -16     
===========================================
- Hits         7838     6308     -1530     
- Misses        150     1664     +1514     

Flag	Coverage Δ
DI	81.67% <0.00%> (-17.14%)	⬇️
DIT	72.71% <0.00%> (-23.68%)	⬇️

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