Rollup merge of #139351 - EnzymeAD:autodiff-batching2, r=oli-obk

Autodiff batching2

~I will rebase it once my first PR landed.~ done.
This autodiff batch mode is more similar to scalar autodiff, since it still only takes one shadow argument.
However, that argument is supposed to be `width` times larger.

r? `@oli-obk`

Tracking:

- rust-lang/rust#124509

Author: matthiaskrgr