Automatic differentiation

Most of the package is compatible with autodiff in Flux and Zygote. Exceptions:

• may or may not work with FFT convolutions. set alg = :direct if encountering autodiff error
• some special border options

An alternative autodiff package is Enzyme.jl which works at the LLVM level and can handle limitations in Zygote eg array mutation. It's probably the future of autodiff in Julia/C++ but isn't yet mature.