fix: hotfix Zygote backend of DifferentiationInterface

[MilesCranmer]

DifferentiationInterface introduced a change in behavior where the Zygote backend returning nothing is now an error rather than nothing; this implements a hotfix. It's not a great solution but I'd prefer this over another extension.

FYI @gdalle I think the ZygoteNothingError should have been introduced in a breaking release as normally it's common to handle nothing explicitly. Though I do think it's the right behavior though overall. Actually changed my mind, I think the nothing should just be returned.

Also would be nice to have ZygoteNothingError defined inside the package itself. If not wanting to put it there, perhaps you could have some trait like:

nothing_error_type(_) = Nothing


# in Zygote extension:
nothing_error_type(::AutoZygote) = ZygoteNothingError

then I can simply call e isa nothing_error_type(backend).

Or maybe have an abstract type of error for when a gradient isn't defined?

[github-actions]

Benchmark Results

	master	146f49d...	master/146f49ddd8031f...
search/multithreading	17 ± 0.97 s	17.2 ± 1.1 s	0.989
search/serial	29.6 ± 0.66 s	29.7 ± 0.85 s	0.994
utils/best_of_sample	1.79 ± 1 μs	1.59 ± 0.52 μs	1.13
utils/check_constraints_x10	12 ± 3.3 μs	12.1 ± 3.4 μs	0.992
utils/compute_complexity_x10/Float64	2.18 ± 1.5 μs	2.18 ± 1.6 μs	1
utils/compute_complexity_x10/Int64	2.11 ± 0.1 μs	2.1 ± 1.6 μs	1
utils/compute_complexity_x10/nothing	1.52 ± 1.5 μs	1.47 ± 1.6 μs	1.03
utils/insert_random_op_x10	6.09 ± 2.7 μs	5.74 ± 1.9 μs	1.06
utils/next_generation_x100	0.405 ± 0.087 ms	0.359 ± 0.1 ms	1.13
utils/optimize_constants_x10	0.037 ± 0.0087 s	0.036 ± 0.0083 s	1.03
utils/randomly_rotate_tree_x10	5.37 ± 0.7 μs	5.43 ± 0.73 μs	0.989
time_to_load	1.82 ± 0.043 s	1.81 ± 0.019 s	1

Benchmark Plots

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Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).

[coveralls]

Pull Request Test Coverage Report for Build 13081620250

Details

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 94.79%

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Totals
Change from base Build 12680128577:	0.004%
Covered Lines:	3166
Relevant Lines:	3340

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💛 - Coveralls

[MilesCranmer]

Fixed at the source with JuliaDiff/DifferentiationInterface.jl#714

[gdalle]

JuliaRegistries/General#124177

[MilesCranmer]

Thanks!