add sret handling for scalar autodiff r? `@oli-obk` Fixing one of the todo's which I left in my previous batching PR. This one handles sret for scalar autodiff. `sret` mostly shows up when we try to return a lot of scalar floats. People often start testing autodiff which toy functions which just use a few scalars as inputs and outputs, and those were the most likely to be affected by this issue. So this fix should make learning/teaching hopefully a bit easier. Tracking: - https://github.com/rust-lang/rust/issues/124509