Properly set within_autodiff (#442)

[jumerckx]

fixes #442
needs Enzyme.jl: EnzymeAD/Enzyme.jl#2254

I had to introduce a new function call_with_reactant_within_autodiff to smuggle the within_autodiff in the call_with_reactant_generator through the self argument.
I also tried doing things through set_reactant_abi but that didn't seem to suffice (first commit).
Perhaps the extra code in set_reactant_abi isn't strictly necessary now so I can try removing it again if wanted.

[wsmoses]

I feel like a cleaner way to do this, is not to have a second interpreter. But instead we can create a new global ref set to false, and overlay within_autodiff to lookup that var, and during autodiff set that to true

[wsmoses]

I'm okay with this though, but if we were to do it in this form, I would probably change call_with_reactant to take a config type var, which stores the current state of whether in autodiff or not (and also we can extend to other things down the line as well)

[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.56%. Comparing base (71b744e) to head (2a4d0f2).

Files with missing lines	Patch %	Lines
src/utils.jl	50.00%	2 Missing ⚠️
src/TracedUtils.jl	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #490      +/-   ##
==========================================
+ Coverage   42.55%   42.56%   +0.01%     
==========================================
  Files         123      123              
  Lines       21816    21826      +10     
==========================================
+ Hits         9283     9290       +7     
- Misses      12533    12536       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[avik-pal]

julia> using Enzyme

julia> function error_not_within_autodiff()
           !Enzyme.within_autodiff() && error("Not within autodiff")
           return nothing
       end
error_not_within_autodiff (generic function with 1 method)

julia> fwd_within_autodiff(Mode, RT) = Enzyme.autodiff(Mode, error_not_within_autodiff, RT)
fwd_within_autodiff (generic function with 1 method)

julia> error_not_within_autodiff()
ERROR: Not within autodiff
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] error_not_within_autodiff()
   @ Main ./REPL[5]:2
 [3] top-level scope
   @ REPL[7]:1
 [4] top-level scope
   @ none:1

julia> fwd_within_autodiff(Forward, Const)
()

julia> error_not_within_autodiff()

julia> Enzyme.within_autodiff()
false

I am extremely confused why is the 2nd call not throw an error here. Only happens if I call fwd_within_autodiff in between. cc @wsmoses this is in isolation from Reactant

[wsmoses]

@vchuravy er wat