Duplicated for `mutable` and `Dual` for forward.

[vchuravy]

Right now the user can accidentally pass in Duplicated(1.0, 0.0) to autodiff, which is odd and surprising since duplicated is for the shadow storage loacation and 0.0 is immutable and can't be changed on the Julia semantics level. We should use a separate type e.g. Dual for forward diff.

[ayeankit]

hey @vchuravy , Is this issue still pending? Can I work on it?

[vchuravy]

Yes, it should be straightforward but will require updating the documentation in many places.