Restore argmax and fix AD test

Author: Katharine Hyatt

src/auxiliary/linalg.jl

@@ -54,3 +54,5 @@ end
 
 safesign(s::Real) = ifelse(s < zero(s), -one(s), +one(s))
 safesign(s::Complex) = ifelse(iszero(s), one(s), s / abs(s))
+
+_argmax(f, domain) = argmax(f, domain)

test/ad.jl

@@ -447,7 +447,7 @@ Vlist = ((ℂ^2, (ℂ^3)', ℂ^3, ℂ^2, (ℂ^2)'),
             T <: Complex && remove_svdgauge_depence!(ΔU, ΔV, U, S, V)
             test_rrule(tsvd, C; atol, output_tangent=(ΔU, ΔS, ΔV))
 
-            c, = TensorKit.MatrixAlgebra._argmax(x -> sqrt(dim(x[1])) * maximum(diag(x[2])),
+            c, = TensorKit._argmax(x -> sqrt(dim(x[1])) * maximum(diag(x[2])),
                                                  blocks(S))
             trunc = truncdim(round(Int, 2 * dim(c)))
             U, S, V = tsvd(C; trunc)