Rollup merge of #139225 - EnzymeAD:rust-lang-enzyme, r=jieyouxu

move autodiff from EnzymeAD/Enzyme to our rust-lang/Enzyme soft-fork

In https://github.com/rust-lang/team/pull/1665 we decided to have a soft-fork of Enzyme, to be able to merge e.g. CI, docs, or cmake improvements if upstream is not interested in them. We don't intend to merge logic/code changes. It also seems generally preferable if we clone submodules from a rust-lang repo, instead of an external org. We don't build or checkout Enzyme yet by default, so this should be a safe rollup.