This is a minimal (<100sloc) package which enables you to add custom gradients to Zygote, without depending on Zygote itself.
Usage:
foo(a, b) = a*b
using ZygoteRules
@adjoint foo(a, b) = a*b, c̄ -> (b'c̄, a'c̄)See the Zygote docs for more details on how to write custom gradients.