Fix _densification_pullback bug

[mzgubic]

Found in JuliaGaussianProcesses/LinearMixingModels.jl#51

Previously we had

julia> test_rrule(Matrix, D, output_tangent=UpperTriangular(rand(4, 4)))
test_rrule: Matrix on BlockDiagonal{Float64, Matrix{Float64}}: Error During Test at /Users/mzgubic/.julia/environments/v1.7/dev/ChainRulesTestUtils/src/testers.jl:193
  Got exception outside of a @test
  MethodError: no method matching _densification_pullback(::UpperTriangular{Float64, Matrix{Float64}}, ::Type{BlockDiagonal{Float64, Matrix{Float64}}}, ::Vector{Int64}, ::Vector{Int64})
  Closest candidates are:
    _densification_pullback(::ChainRulesCore.AbstractThunk, ::Any, ::Any, ::Any) at ~/JuliaEnvs/LinearMixingModels.jl/dev/BlockDiagonals/src/chainrules.jl:39
    _densification_pullback(::Matrix, ::Any, ::Any, ::Any) at ~/JuliaEnvs/LinearMixingModels.jl/dev/BlockDiagonals/src/chainrules.jl:28

This fixes the bug

[codecov]

Codecov Report

Merging #111 (e7f8c20) into master (7bac0da) will decrease coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
- Coverage   96.17%   95.98%   -0.19%     
==========================================
  Files           5        5              
  Lines         340      324      -16     
==========================================
- Hits          327      311      -16     
  Misses         13       13              

Impacted Files	Coverage Δ
src/chainrules.jl	100.00% <100.00%> (ø)
src/blockdiagonal.jl	86.76% <0.00%> (-0.56%)	⬇️
src/linalg.jl	95.34% <0.00%> (-0.21%)	⬇️
src/base_maths.jl	100.00% <0.00%> (ø)

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