implement some chainrules

[lazarusA]

maybe defining a few chainrules for DD, similar to

https://github.com/mcabbott/AxisKeys.jl/blob/master/src/chainrules.jl

would be useful for a wider adoption of DD.

[rafaqz]

I guess we can just duplicate these tests?

https://github.com/mcabbott/AxisKeys.jl/blob/master/test/_chainrules.jl

I dont understand differentiation or its package ecosystem like invenia people do, but I can copy their code 😅

[sethaxen]

I'd be happy to help with this. While ChainRulesCore was originally intended to be a lightweight dep, it's more commonly now used as a weak dep in an extension.