`CombinedAdjointThunk` should also return ret value

[vchuravy]

julia> autodiff(f, Active(2.0))
(4.0,)

Should maybe be:

julia> (4.0, (4.0,))
(4.0, (4.0,))

[vchuravy]

https://github.com/wsmoses/Enzyme.jl/blob/aa78016bfd046689aa33b2ceba50049b329dd0d5/src/compiler.jl#L1189

[vchuravy]

@tansongchen this might be a good place to start.

[tansongchen]

Just went though compiler.jl, this should be straightfoward. will do it

[wsmoses]

Adding this as an option actually enables us to handle https://github.com/wsmoses/Enzyme.jl/issues/112 by simply returning the shadow return via the corresponding shadow return arg.

[vchuravy]

@wsmoses #537 did implement this right?

[wsmoses]

I don't think so, but it changed the ABI to have a place for this.

[wsmoses]

Done in https://github.com/EnzymeAD/Enzyme.jl/pull/598