As discussed on Slack, it is sometimes annoying to use reverse mode because you need autodiff_thunk whenever

• the function output is a number and the cotangent is different from 1.0
• the function output is anything but a number

A solution suggested by @wsmoses would be syntactic sugar for vjp (and maybe jvp but that is less necessary cause autodiff gets the job done every time). There are a few design questions around this:

• Should we put dreturn together with the return activity?
  • Good: 1 argument instead of 2
  • Bad: in the case of Duplicated returns, we would have to specify dreturn = Duplicated(whatever, actual_thing_we_care_about).
• Can we generalize this to batch mode vjp? How do we pass several cotangents if the return value is Active?