Fix type of AbstractFFTs.Plan for real-complex FFTs - #2504

using CUDA
using FFTW
using AbstractFFTs

T = Float64

# CPU

u = rand(T, 256)
p_cpu = plan_rfft(u)
v = p_cpu * u
w = p_cpu \ v

p_cpu isa AbstractFFTs.Plan{T}  # true
p_cpu.pinv isa AbstractFFTs.Plan{Complex{T}}  # true

eltype(p_cpu)       # Float64
eltype(p_cpu.pinv)  # ComplexF64

# GPU

u_gpu = CuArray(u)
p_gpu = plan_rfft(u_gpu)
v_gpu = p_gpu * u_gpu
w_gpu = p_gpu \ v_gpu

p_gpu isa AbstractFFTs.Plan{T}  # false
p_gpu.pinv isa AbstractFFTs.Plan{Complex{T}}  # false

eltype(p_gpu)       # ComplexF64
eltype(p_gpu.pinv)  # Float64