[CarloLucibello]

Closes #860.

The pullbacks of reduce(vcat, As), reduce(hcat, As) (reduce_vcat_pullback, reduce_hcat_pullback_1, reduce_hcat_pullback_2) and hvcat (hvcat_pullback) index the cotangent once per input. Indexing an AbstractThunk calls unthunk each time, so a thunked cotangent (e.g. from sum's pullback in Zygote, FluxML/Zygote.jl#1686) is recomputed length(As) times. This calls unthunk once at the top of each, as hcat/vcat/cat/stack already do.

I used dY = unthunk(ȳ) (renaming the argument) rather than dY = unthunk(dY), matching the neighbouring pullbacks and avoiding rebinding a variable captured by the do blocks. hvcat_pullback has the same pattern and is included.

Measured on Julia 1.13.2 (min of 5 runs), thunked cotangent @thunk(ones(size(Y))):

pullback	before	after	plain array
reduce(vcat), 1600 × Vector(10)	3.9 ms, 196 MiB	0.026 ms, 0.35 MiB	0.024 ms
same, InplaceableThunk	5.8 ms, 196 MiB	0.030 ms, 0.35 MiB	0.023 ms
reduce(vcat), 800 × Matrix(10×2)	1.5 ms, 98 MiB	0.026 ms, 0.31 MiB	0.017 ms
reduce(hcat), 1600 × Vector(10)	4.3 ms, 196 MiB	0.024 ms, 0.35 MiB	0.024 ms
reduce(hcat), 800 × Matrix(10×2)	1.6 ms, 98 MiB	0.025 ms, 0.31 MiB	0.020 ms
hvcat, 200 × Vector(10)	0.145 ms, 3.1 MiB	0.010 ms, 0.05 MiB	0.003 ms

Added a testset in test/rulesets/Base/array.jl checking that @thunk and InplaceableThunk cotangents give the same result as the plain array for all the variants above (vectors / matrices / mixed), and that @allocated back(thunk) < 10 * sizeof(Y) at 1600 inputs (the quadratic path allocates about length(As) * sizeof(Y)). Without the fix, the 10 allocation checks fail and the 14 equality checks pass; with it all 24 pass, also under --check-bounds=yes --code-coverage=user.

Tests: test/runtests.jl rulesets/Base on Julia 1.13.2 passes (16978 pass, 52 broken as before). Not run locally on 1.10 (left to CI).

Bumps the patch version to 1.74.10, as recent PRs do.

Noticed but left alone:

• vect pullbacks index ȳ[n] per element and are equally quadratic in the number of arguments (N=256: 0.8 ms vs 3 µs), but N is a compile-time argument count, so I did not touch them.
• Other pullbacks I looked at in src/rulesets/Base/ either already unthunk or access the cotangent once.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.64%. Comparing base (a441bd5) to head (d32589b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #861   +/-   ##
=======================================
  Coverage   96.63%   96.64%           
=======================================
  Files          28       28           
  Lines        3598     3602    +4     
=======================================
+ Hits         3477     3481    +4     
  Misses        121      121           

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