Formalize backend interface

[gdalle]

DI defines several backend objects and wrappers (in addition to those from ADTypes):

• SecondOrder
• MixedMode
• AutoForwardFromPrimitive, AutoReverseFromPrimitive
• AutoZeroForward, AutoZeroReverse
• AutoSimpleFiniteDiff

For each of those, several functions need to be defined:

• ADTypes.mode
• DI.check_available
• DI.inplace_support
• DI.pick_batchsize (for integers and for arrays), and DI.threshold_batchsize for testing
• DI.inner_preparation_behavior, and DI.overloaded_input if necessary

This should all be documented and properly tested.