Fix QR and LQ pullback for rank 0 blocks (#223)

* Replace `findlast` with `count`

* replace count with searchsortedlast

* Revert back to `findlast` for QR and LQ

Author: lkdvos

ext/TensorKitChainRulesCoreExt/factorizations.jl

@@ -219,7 +219,7 @@ function svd_pullback!(ΔA::AbstractMatrix, U::AbstractMatrix, S::AbstractVector
     Sp = view(S, 1:p)
 
     # rank
-    r = count(>(tol), S)
+    r = searchsortedlast(S, tol; rev=true)
 
     # compute antihermitian part of projection of ΔU and ΔV onto U and V
     # also already subtract this projection from ΔU and ΔV
@@ -376,7 +376,7 @@ end
 function qr_pullback!(ΔA::AbstractMatrix, Q::AbstractMatrix, R::AbstractMatrix, ΔQ, ΔR;
                       tol::Real=default_pullback_gaugetol(R))
     Rd = view(R, diagind(R))
-    p = findlast(>=(tol) ∘ abs, Rd)
+    p = something(findlast(≥(tol) ∘ abs, Rd), 0)
     m, n = size(R)
 
     Q1 = view(Q, :, 1:p)
@@ -427,7 +427,7 @@ end
 function lq_pullback!(ΔA::AbstractMatrix, L::AbstractMatrix, Q::AbstractMatrix, ΔL, ΔQ;
                       tol::Real=default_pullback_gaugetol(L))
     Ld = view(L, diagind(L))
-    p = findlast(>=(tol) ∘ abs, Ld)
+    p = something(findlast(≥(tol) ∘ abs, Ld), 0)
     m, n = size(L)
 
     L1 = view(L, :, 1:p)