[NDTensors] [BUG] `similartype(::Diagonal)` returns `Vector` instead of `Matrix`

[mtfishman]

For example:

julia> using LinearAlgebra: Diagonal

julia> using NDTensors.TypeParameterAccessors: similartype

julia> similartype(Diagonal{Float64,Vector{Float64}})
Vector{Float64} (alias for Array{Float64, 1})

Hit this in #1560.