Fix vcat method ambiguity issues on Julia 1.8

[devmotion]

Fixes the vcat test errors due to a method ambiguity issue on Julia 1.8. These test errors are present on the master branch and show also up in other unrelated PRs.

Maybe at some point one should clean up the vcat/hcat/cat definitions, similar to FluxML/Tracker.jl#128. But for now it seemed easier to just fix the reported method ambiguity issue.

[devmotion]

The test errors are known and caused by the latest release of DiffTests. I opened an issue and a PR a week ago.

[mohdibntarek]

Can we upper bound the version of DiffTests?

[devmotion]

A bit unfortunate but I guess that's probably the fastest and easiest solution for now. I also checked JuliaDiff/DiffTests.jl#15 with this PR locally, and while it seems to fix the current issues it still fails due to some newly uncovered ambiguity issues (e.g., *(::Diagonal, ::TrackedMatrix)).

[codecov-commenter]

Codecov Report

Base: 85.09% // Head: 82.19% // Decreases project coverage by -2.89% ⚠️

Coverage data is based on head (f5710a6) compared to base (4a50321).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
- Coverage   85.09%   82.19%   -2.90%     
==========================================
  Files          18       18              
  Lines        1811     1528     -283     
==========================================
- Hits         1541     1256     -285     
- Misses        270      272       +2     

Impacted Files	Coverage Δ
src/derivatives/arrays.jl	92.85% <ø> (+1.68%)	⬆️
src/derivatives/broadcast.jl	76.62% <0.00%> (-13.94%)	⬇️
src/tape.jl	55.55% <0.00%> (-9.03%)	⬇️
src/api/hessians.jl	84.00% <0.00%> (-3.50%)	⬇️
src/tracked.jl	88.42% <0.00%> (-3.48%)	⬇️
src/api/tape.jl	72.22% <0.00%> (-2.78%)	⬇️
src/api/utils.jl	84.84% <0.00%> (-2.06%)	⬇️
src/derivatives/elementwise.jl	77.48% <0.00%> (-1.33%)	⬇️
src/derivatives/linalg/reductions.jl	70.31% <0.00%> (-1.30%)	⬇️
src/derivatives/scalars.jl	95.74% <0.00%> (-0.98%)	⬇️
... and 8 more

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[devmotion]

As expected, with DiffTests 0.1.1 all tests pass.