chore: add symmteric accum method for empty gradients (#1577)

Author: DhairyaLGandhi

src/lib/lib.jl

@@ -16,6 +16,7 @@ accum(x, y, zs...) = accum(accum(x, y), zs...)
 accum(x::Tuple, ys::Tuple...) = map(accum, x, ys...)
 accum(x::AbstractArray, ys::AbstractArray...) = Base.broadcast_preserving_zero_d(accum, x, ys...)
 accum(::Tuple{}, ::NamedTuple{}) = ()
+accum(::NamedTuple{}, ::Tuple{}) = ()
 
 @generated function accum(x::NamedTuple, y::NamedTuple)
   # assumes that y has no keys apart from those also in x