MethodError: (::ProjectTo{NoTangent, @NamedTuple{}})(::Thunk) is ambiguous

[cossio]

julia> using Zygote, ChainRules, ChainRulesCore

julia> x = BitArray([0])
julia> _, back = rrule(AbstractArray{Float64}, x)
julia> (::ProjectTo{NoTangent})(::AbstractThunk) = NoTangent()

julia> back([1.0])
(NoTangent(), NoTangent())

julia> back(@thunk([1.0]))
ERROR: MethodError: (::ProjectTo{NoTangent, @NamedTuple{}})(::Thunk{var"#1#2"}) is ambiguous.

CC @oxinabox @mcabbott @ToucheSir

From a conversation on Slack, the fix seems to be to define the method:

(::ProjectTo{NoTangent}(::Thunk) = NoTangent()

[oxinabox]

You also need

(::ProjectTo{NoTangent}(::AbstractThunk) = NoTangent()

So that the sane thing bug written with
InplaceableThunks doesn't have the same problem.

---

This should give a performance win,
because it avoids unthunking

[cossio]

@oxinabox I made a PR, could you please review it and merge ?