Remove `stack`-based out-of-place Jacobian and Hessian?

[gdalle]

Currently, out-of-place Jacobians and Hessians use stack to put together the batches of JVPs/VJPs/HVPs, and there is a possible resizing at the end.

• Advantages: preserves the native types of matrix-vector products
• Drawbacks: can cause type instability (in the closure) and unwanted allocations (when concatenating or resizing)

Is there any drawback to defining a buffer as jac = similar(x, length(y), length(x)) or hess = similar(x, length(x), length(x))?

The only one I see is that the operators would turn SArrays into MArrays

[gdalle]

Closing because it would impose mutability where I've made sure to avoid it