start testing Enzyme

[CarloLucibello]

We don't officially support Enzyme yet, but since we are going to in the future we better start testing, see what is missing, and watch out for regressions.
The testing work has already started at EnzymeAD/Enzyme.jl#805, here I mostly copy some of the tests contributed by @jgreener64 so that they become part of Flux's testsuite.

Related issues in Enzyme.jl:

• Discussion: what does Enzyme.jl need for DL in Julia? EnzymeAD/Enzyme.jl#805
• Error with Flux GRU layer EnzymeAD/Enzyme.jl#1261
• fail with simple sum gradient on CUDA EnzymeAD/Enzyme.jl#1347

PRs in CUDA.jl:

• Add Enzyme Forward mode custom rule JuliaGPU/CUDA.jl#1869

[CarloLucibello]

if tests pass as they should this can be merged.

This PR gives us already some good cpu coverage. For CUDA instead, I get some pretty fundamental problems at the moment EnzymeAD/Enzyme.jl#1347. GPU tests will be added in future PRs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.73%. Comparing base (5f84b68) to head (767349e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2392      +/-   ##
==========================================
+ Coverage   43.04%   45.73%   +2.68%     
==========================================
  Files          32       32              
  Lines        1856     1863       +7     
==========================================
+ Hits          799      852      +53     
+ Misses       1057     1011      -46     

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[CarloLucibello]

ready to merge

[CarloLucibello]

bump