LQ pullback rules aren't GPU friendly

[kshyatt]

https://github.com/QuantumKitHub/MatrixAlgebraKit.jl/blob/main/src/pullbacks/lq.jl#L93 offending line is here, the stacktrace is:

  If you want to allow scalar iteration, use `allowscalar` or `@allowscalar`                                                                                                                                    
  to enable scalar iteration globally or for the operations in question.                                                                                                                                        
  Stacktrace:                                                                                                                                                                                                   
    [1] error(s::String)                                                                                                                                                                                        
      @ Base ./error.jl:44                                                                                                                                                                                      
    [2] errorscalar(op::String)                                                                                                                                                                                 
      @ GPUArraysCore ~/.julia/packages/GPUArraysCore/aNaXo/src/GPUArraysCore.jl:151                                                                                                                            
    [3] _assertscalar(op::String, behavior::GPUArraysCore.ScalarIndexing)                                                                                                                                       
      @ GPUArraysCore ~/.julia/packages/GPUArraysCore/aNaXo/src/GPUArraysCore.jl:124                                                                                                                            
    [4] assertscalar(op::String)                                                                                                                                                                                
      @ GPUArraysCore ~/.julia/packages/GPUArraysCore/aNaXo/src/GPUArraysCore.jl:112                                                                                                                            
    [5] getindex                                                                                                                                                                                                
      @ ~/.julia/packages/GPUArrays/DPs2B/src/host/indexing.jl:50 [inlined]                                                                                                                                     
    [6] getindex                                                                                                                                                                                                
      @ ~/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/share/julia/stdlib/v1.12/LinearAlgebra/src/diagonal.jl:182 [inlined]                                                                                      
    [7] getindex                                                                                                                                                                                                
      @ ./subarray.jl:316 [inlined]                                                                                                                                                                             
    [8] getindex                                                                                                                                                                                                
      @ ~/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/share/julia/stdlib/v1.12/LinearAlgebra/src/triangular.jl:243 [inlined]                                                                                    
    [9] macro expansion                                                                                                                                                                                         
      @ ~/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/share/julia/stdlib/v1.12/LinearAlgebra/src/generic.jl:100 [inlined]                                                                                          [10] _generic_matmatmul_nonadjtrans!(C::CUDACore.CuArray{Float64, 2, CUDACore.DeviceMemory}, A::LinearAlgebra.UpperTriangular{Float64, LinearAlgebra.Adjoint{Float64, SubArray{Float64, 2, LinearAlgebra.Diagonal{Float64, CUDACore.CuArray{Float64, 1, CUDACore.DeviceMemory}}, Tuple{UnitRange{Int64}, UnitRange{Int64}}, false}}}, B::LinearAlgebra.LowerTriangular{Float64, SubArray{Float64, 2, LinearAlgebra.Diagonal{Float64, CUDACore.CuArray{Float64, 1, CUDACore.DeviceMemory}}, Tuple{UnitRange{Int64}, UnitRange{Int64}}, false}}, alpha::Int64, beta::Int64)                                                                    
      @ LinearAlgebra ~/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/share/julia/stdlib/v1.12/LinearAlgebra/src/matmul.jl:1033                                                                                   
   [11] __generic_matmatmul!                                                                                                                                                                                    
      @ ~/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/share/julia/stdlib/v1.12/LinearAlgebra/src/matmul.jl:1025 [inlined]                                                                                       
   [12] _generic_matmatmul!(C::CUDACore.CuArray{Float64, 2, CUDACore.DeviceMemory}, A::LinearAlgebra.UpperTriangular{Float64, LinearAlgebra.Adjoint{Float64, SubArray{Float64, 2, LinearAlgebra.Diagonal{Float64, CUDACore.CuArray{Float64, 1, CUDACore.DeviceMemory}}, Tuple{UnitRange{Int64}, UnitRange{Int64}}, false}}}, B::LinearAlgebra.LowerTriangular{Float64, SubArray{Float64, 2, LinearAlgebra.Diagonal{Float64, CUDACore.CuArray{Float64, 1, CUDACore.DeviceMemory}}, Tuple{UnitRange{Int64}, UnitRange{Int64}}, false}}, alpha::Int64, beta::Int64)                                                                                
      @ LinearAlgebra ~/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/share/julia/stdlib/v1.12/LinearAlgebra/src/matmul.jl:1019                                                                                   
   [13] generic_matmatmul!                                                                                                                                                                                      
      @ ~/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/share/julia/stdlib/v1.12/LinearAlgebra/src/matmul.jl:1009 [inlined]                                                                                       
   [14] _mul!                                                                                                                                                                                                   
      @ ~/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/share/julia/stdlib/v1.12/LinearAlgebra/src/triangular.jl:1159 [inlined]                                                                                   
   [15] mul!                                                                                                                                                                                                    
      @ ~/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/share/julia/stdlib/v1.12/LinearAlgebra/src/matmul.jl:297 [inlined]                                                                                        
   [16] lq_pullback!(ΔA::LinearAlgebra.Diagonal{Float64, CUDACore.CuArray{Float64, 1, CUDACore.DeviceMemory}}, A::LinearAlgebra.Diagonal{Float64, CUDACore.CuArray{Float64, 1, CUDACore.DeviceMemory}}, LQ::Tuple{LinearAlgebra.Diagonal{Float64, CUDACore.CuArray{Float64, 1, CUDACore.DeviceMemory}}, LinearAlgebra.Diagonal{Float64, CUDACore.CuArray{Float64, 1, CUDACore.DeviceMemory}}}, ΔLQ::Tuple{LinearAlgebra.Diagonal{Float64, CUDACore.CuArray{Float64, 1, CUDACore.DeviceMemory}}, LinearAlgebra.Diagonal{Float64, CUDACore.CuArray{Float64, 1, CUDACore.DeviceMemory}}}; rank_atol::Float64, gauge_atol::Float64)                 
      @ MatrixAlgebraKit ~/.julia/dev/MatrixAlgebraKit/src/pullbacks/lq.jl:93  

This one might also need an upstream fix at GPUArrays.jl