frule_via_ad should accept several arguments

[gdalle]

According to the official API specification of ChainRulesCore.jl, frule_via_ad should accept all the arguments of the function as a destructured tuple: https://juliadiff.org/ChainRulesCore.jl/stable/api.html#ChainRulesCore.frule_via_ad

However, it seems that ForwardDiffChainRules.jl only accepts one argument:

ForwardDiffChainRules.jl/src/ForwardDiffChainRules.jl

Lines 46 to 52 in 609201f

	function ChainRulesCore.frule_via_ad(
	config::ForwardDiffRuleConfig,
	tangent,
	f,
	f_arg;
	kwargs...,
	)

I think this is the reason for a bug in my code. Do you think it is fixable?

[gdalle]

@mohamed82008 any idea?

[mohamed82008]

Not a fan of supporting multiple arguments with ForwardDiff. ForwardDiff's API does not support that.

[mohamed82008]

hmm on second thought, I changed my mind

[gdalle]

Given the metaprogramming-heavy nature of this package, I'm not sure how to make a PR implementing this

[mohamed82008]

NonconvexUtils already supports this https://github.com/JuliaNonconvex/NonconvexUtils.jl/blob/main/src/forwarddiff_frule.jl#L1. You can just assume the inputs are all real/arrays and not recursive containers and simplify the implementation. I use flatten/unflatten but that's because I wanted to be too generic. No need to be that generic.