rrule for map for tuples is outdated with respect to JuliaLang/julia#42216

[nsajko]

ChainRules.jl/src/rulesets/Base/base.jl

Lines 243 to 273 in 9dd39bd

	function rrule(config::RuleConfig{>:HasReverseMode}, ::typeof(map), f::F, xs::Tuple...) where {F}
	length_y = minimum(length, xs)
	hobbits = ntuple(length_y) do i
	args = getindex.(xs, i)
	rrule_via_ad(config, f, args...)
	end
	y = map(first, hobbits)
	num_xs = Val(length(xs))
	paddings = map(x -> ntuple(Returns(NoTangent()), (length(x) - length_y)), xs)
	all(isempty, paddings) || @error """map(f, xs::Tuple...) does not allow mistmatched lengths!
	But its `rrule` does; when JuliaLang/julia #42216 is fixed this warning should be removed."""
	function map_pullback(dy_raw)
	dy = unthunk(dy_raw)
	# We want to call the pullbacks in `rrule_via_ad` in reverse sequence to the forward pass:
	backevals = ntuple(length_y) do i
	rev_i = length_y - i + 1
	last(hobbits[rev_i])(dy[rev_i])
	end |> reverse
	# This df doesn't infer, could test Base.issingletontype(F), but it's not the only inference problem.
	df = ProjectTo(f)(sum(first, backevals))
	# Now unzip that. Because `map` like `zip` should when any `x` stops, some `dx`s may need padding.
	# Although in fact, `map(+, (1,2), (3,4,5))` is an error... https://github.com/JuliaLang/julia/issues/42216
	dxs = ntuple(num_xs) do k
	dx_short = map(bv -> bv[k+1], backevals)
	ProjectTo(xs[k])((dx_short..., paddings[k]...)) # ProjectTo makes the Tangent for us
	end
	return (NoTangent(), df, dxs...)
	end
	map_back(dy::AbstractZero) = (NoTangent(), NoTangent(), ntuple(Returns(NoTangent()), num_xs)...)
	return y, map_pullback
	end

The referenced Julia issue is resolved: JuliaLang/julia#42216

[oxinabox]

Should be an easy PR to fix.

1. Add a test that hits it
2. delete the warning