remove duplicate _diagm_back definition

Author: ElOceanografo

src/rulesets/LinearAlgebra/structured.jl

@@ -96,7 +96,7 @@ end
 
 function _diagm_back(p, ȳ)
     k, v = p
-    d = diag(unthunk(ȳ), k)[1:length(v)] # handle if diagonal was smaller than matrix
+    d = diag(unthunk(ȳ), k)[eachindex(v)] # handle if diagonal was smaller than matrix
     return Tangent{typeof(p)}(second = d)
 end
 

src/rulesets/SparseArrays/sparsematrix.jl

@@ -160,10 +160,3 @@ function rrule(::typeof(spdiagm), v::AbstractVector)
     end
     return spdiagm(v), spdiagm_pullback
 end
-
-
-function _diagm_back(p, ȳ)
-    k, v = p
-    d = diag(unthunk(ȳ), k)[eachindex(v)]  # handle if diagonal was smaller than matrix
-    return Tangent{typeof(p)}(second = d)
-end