New Zygote context in every call to AD.pullback_function

[devmotion]

Alternative to #76 that I had in mind when writing #76 (review).

In contrast to #76, with this PR ZygoteBackend would still be a ReverseRuleConfigBackend, and hence the behaviour of ZygoteBackend and ReverseRuleConfigBackend(Zygote.ZygoteRuleConfig()) would not differ.

Fixes #69. Closes #70. Closes #76.

[codecov]

Codecov Report

Patch coverage: 75.00% and project coverage change: -0.12 ⚠️

Comparison is base (19ce815) 84.36% compared to head (848ee38) 84.25%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
- Coverage   84.36%   84.25%   -0.12%     
==========================================
  Files           8        8              
  Lines         467      470       +3     
==========================================
+ Hits          394      396       +2     
- Misses         73       74       +1     

Impacted Files	Coverage Δ
src/backends.jl	83.33% <0.00%> (-16.67%)	⬇️
ext/AbstractDifferentiationChainRulesCoreExt.jl	100.00% <100.00%> (ø)
ext/AbstractDifferentiationZygoteExt.jl	100.00% <100.00%> (ø)

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[devmotion]

I made the fix more modular in 848ee38, to avoid code duplication and make it easy to apply similar fixes for other AD backends if needed.

[ToucheSir]

Well, that eliminates my only substantive point in #76 (comment). As the adage goes, the fastest way to get something done is to do it wrong and get someone to correct it for you 😆

[devmotion]

That is, you approve this PR? 🙂

[ToucheSir]

Yes, though I'm not sure how much weight it carries here :)

[devmotion]

I trust your Zygote expertise 🙂