Problem with `Base.mapreducedim!`

## Problem 1
method ambiguity for Adjoint
```julia
julia> Base.mapreducedim!(identity, +, CUDA.zeros(4), CUDA.ones(4,4)')
ERROR: MethodError: mapreducedim!(::typeof(identity), ::typeof(+), ::CuArray{Float32, 1, CUDA.Mem.DeviceBuffer}, ::Adjoint{Float32, CuArray{Float32, 2, CUDA.Mem.DeviceBuffer}}) is ambiguous.

Candidates:
mapreducedim!(f, op, R::AnyGPUArray, A::AbstractArray)
@ GPUArrays ~/.julia/packages/GPUArrays/Hd5Sk/src/host/mapreduce.jl:10
mapreducedim!(f, op::Union{typeof(&), typeof(+), typeof(Base._extrema_rf), typeof(Base.add_sum), typeof(max), typeof(min), typeof(|)}, B::AbstractArray, A::Adjoint{T, <:AbstractMatrix} where T)
@ LinearAlgebra ~/.julia/juliaup/julia-1.10.2+0.x64.linux.gnu/share/julia/stdlib/v1.10/LinearAlgebra/src/adjtrans.jl:427

Possible fix, define
mapreducedim!(::Any, ::Union{typeof(&), typeof(+), typeof(Base._extrema_rf), typeof(Base.add_sum), typeof(max), typeof(min), typeof(|)}, ::AnyGPUArray, ::Adjoint{T, <:AbstractMatrix} where T)

Stacktrace:
[1] top-level scope
@ REPL[39]:1
[2] top-level scope
@ ~/.julia/packages/CUDA/htRwP/src/initialization.jl:206
```

## Problem 2
not dropping singleton dims
```julia
julia> Base.mapreducedim!(identity, +, CUDA.zeros(4), CUDA.ones(4,4,4))
4×1×1 CuArray{Float32, 3, CUDA.Mem.DeviceBuffer}:
[:, :, 1] =
16.0
16.0
16.0
16.0

julia> Base.mapreducedim!(identity, +, zeros(4), ones(4,4,4))
4-element Vector{Float64}:
16.0
16.0
16.0
16.0
```

Reproduce both examples in the issue, then read host/mapreduce.jl and the referenced LinearAlgebra/src/adjtrans.jl method. Compare the GPU and CPU mapreducedim! results, focusing on the Adjoint ambiguity and singleton dimensions. Done means the ambiguous call dispatches successfully and the GPU result has the same dimensional shape as the CPU result.

Written by the indexing model from the issue text.