macro for importing rule from chain rules for all backends that support it and are loaded

[CarloLucibello]

A few backends provide support for rules from chainrules:

• Zygote: does it automatically
• ForwarDiff: CR support through https://github.com/ThummeTo/ForwardDiffChainRules.jl
• ReverseDiff: https://juliadiff.org/ReverseDiff.jl/dev/api/#ReverseDiff.@grad_from_chainrules
• Enzyme: https://enzymead.github.io/Enzyme.jl/dev/api/#Enzyme.@import_rrule-Tuple
• Mooncake: https://chalk-lab.github.io/Mooncake.jl/stable/developer_documentation/internal_docstrings/#Mooncake.@from_chainrules

DI could provide a macro that does the same for all loaded backend that support CR

[yebai]

Just a note in case this is useful to the community, Mooncake rules can be viewed as a prototype for ChainRules2. It introduces two notable improvements:

• support for mutation, and
• uniquely typed tangents, which are essential for exhaustive rule testing.

Both improvements reflect lessons learned from ChainRules by @willtebbutt and @oxinabox.