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Use zygote2differential to wrap chainrules inputs #1057

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Use zygote2differential to wrap chainrules inputs #1057

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8fb20af
Use zygote2differential to wrap chainrules inputs
oxinabox Aug 25, 2021
9ca4402
fix typos and mised bits
oxinabox Aug 25, 2021
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23 changes: 6 additions & 17 deletions src/compiler/chainrules.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -117,27 +117,16 @@ for T_outer in (:Tuple, :NamedTuple)
end

"""
wrap_chainrules_input(x)

Convert `x` from the format Zygote uses internally to differentials types ChainRules uses.
"""
@inline wrap_chainrules_input(x) = x
@inline wrap_chainrules_input(::Nothing) = ChainRules.ZeroTangent()
@inline function wrap_chainrules_input(xs::Union{Tuple, NamedTuple})
xp = map(wrap_chainrules_input, xs)
ChainRules.Tangent{Any, typeof(xp)}(xp)
end

"""
ZBack{F}(back) <: Function
ZBack{F,P}(back::F, primals::P) <: Function

Wrapper for a ChainRules pullback `back`, that causes it to follow Zygote conventions.
(A functor here is used rather than a closure to avoid boxing issues);
"""
struct ZBack{F} <: Function
struct ZBack{F,P} <: Function
back::F
primals::P
end
@inline (s::ZBack)(dy) = wrap_chainrules_output(s.back(wrap_chainrules_input(dy)))
@inline (s::ZBack)(dy) = wrap_chainrules_output(s.back(zygote2differential(dy, s.primals)))
# `nothing->nothing` can be deleted after https://github.com/FluxML/Zygote.jl/issues/603
# though it might be worth keeping as a performance optimization (benchmarking pending)
@inline (s::ZBack)(::Nothing) = nothing
Expand All @@ -150,7 +139,7 @@ The pullback is appropriately wrapped up to follow Zygote conventions.
"""
@inline function chain_rrule(config, f, args...)
y, back = rrule(config, f, args...)
return y, ZBack(back)
return y, ZBack(back, args)
end


Expand All @@ -163,7 +152,7 @@ As per [`chain_rrule`](@ref) but with support for kwargs.
@inline function chain_rrule_kw(config, kwf, kwargs, f, args...)
y, back = rrule(config, f, args...; kwargs...)
function kw_zpullback(dy)
dxs = ZBack(back)(dy)
dxs = ZBack(back, args)(dy)
if dxs === nothing # if dxs is nothing, then all partiaols are nothing
# Zygote convention is a single nothing no mather how partials, if all are nothing
return nothing
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