LexaLutyi

`gradient` returns `nothing` for `CuArrays{ComplexF32}`, but works fine with `Arrays{ComplexF32}`: ``` julia> a = CUDA.rand(ComplexF32, 2) 2-element CuArray{ComplexF32, 1, CUDA.Mem.DeviceBuffer}: 0.040781975f0 + 0.31357694f0im 0.0050712824f0 + 0.464033f0im julia> gradient(t -> sum(abs2,...