Problem with `MultiFESpace` assembly

[bmxam]

The following code

module tmp
using Bcube
using BcubeAccelerated
using KernelAbstractions

mesh = one_cell_mesh(:line)

fs = FunctionSpace(:Lagrange, 1)
U_p = TrialFESpace(fs, mesh)
U_u = TrialFESpace(fs, mesh)

V_p = TestFESpace(U_p)
V_u = TestFESpace(U_u)

V = MultiFESpace(V_p, V_u)

dΩ = Measure(CellDomain(mesh), 1)
l((v_u, v_p)) = ∫(v_p)dΩ

backend = BcubeAccelerated.BcubeBackendAcc(get_backend(ones(2)); kernel=BcubeAccelerated.KernelKA(), sync=false)
y = Bcube.allocate_dofs(V)
assemble_linear!(y, l, V; backend)
end

produces the following error:

ERROR: LoadError: MethodError: no method matching get_backend(::Tuple{SubArray{Float64, 1, Vector{…}, Tuple{…}, false}, SubArray{Float64, 1, Vector{…}, Tuple{…}, false}})
The function `get_backend` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  get_backend(::Bcube.BcubeBackendCPUSerial)
   @ BcubeAccelerated BcubeAccelerated\lh15Z\src\backend.jl:39
  get_backend(::RecursiveArrayTools.VectorOfArray)
   @ RecursiveArrayToolsKernelAbstractionsExt RecursiveArrayTools\jieUI\ext\RecursiveArrayToolsKernelAbstractionsExt.jl:6
  get_backend(::StaticArraysCore.MArray)
   @ KernelAbstractions KernelAbstractions\lGrz7\ext\StaticArraysExt.jl:7
  ...

Stacktrace:
 [1] KA_assemble!(y::Tuple{…}, f::Function, elts::Nothing, V::Bcube.MultiFESpace{…}, measure::Bcube.Measure{…}, backend::BcubeAccelerated.BcubeBackendAcc{…})
   @ BcubeAccelerated BcubeAccelerated\lh15Z\src\assembler.jl:49
 [2] __assemble_linear!(b::Tuple{…}, f::Function, V::Bcube.MultiFESpace{…}, measure::Bcube.Measure{…}, backend::BcubeAccelerated.BcubeBackendAcc{…})
   @ BcubeAccelerated BcubeAccelerated\lh15Z\src\assembler.jl:39
 [3] _assemble_linear!(b::Vector{…}, l::typeof(Main.tmp.l), V::Bcube.MultiFESpace{…}, integration::Bcube.Integration{…}, backend::BcubeAccelerated.BcubeBackendAcc{…})
   @ Bcube Bcube.jl\src\assembler\assembler.jl:241
 [4] assemble_linear!(b::Vector{…}, l::typeof(Main.tmp.l), V::Bcube.MultiFESpace{…}; backend::BcubeAccelerated.BcubeBackendAcc{…})
   @ Bcube Bcube.jl\src\assembler\assembler.jl:220
 [5] top-level scope
   @ BcubeMiniApps\stability\taylor-culick\tmp\tmp.jl:22
 [6] include(fname::String)
   @ Main .\sysimg.jl:38
 [7] top-level scope
   @ REPL[4]:1

This is because we reshape the vector b into a Tuple of SubArraywhen dealing with multi-fespaces. Maybe we should move

BcubeAccelerated.jl/src/assembler.jl

Line 49 in 9d7eefd

@assert backendKA == get_backend(y)

in Bcube, here :
https://github.com/bcube-project/Bcube.jl/blob/a0858c10c9a465720fefa4c9251e38baa82fec0c/src/assembler/assembler.jl#L240