`productMPS` not differentiable

[GTorlai]

sites = siteinds("Qubit", 3)

function f(x)
  ψ = productMPS(sites, "0")
  return x * inner(ψ,ψ)
end

f'(2.0)

Error:

ERROR: TypeError: in Type, in parameter, expected Type, got a value of type ITensor
Stacktrace:
  [1] _pullback
    @ ~/.julia/dev/ITensors/src/physics/sitetype.jl:106 [inlined]
  [2] _pullback(ctx::Zygote.Context, f::Type{SiteType}, args::UInt128)
    @ Zygote ~/.julia/packages/Zygote/Lw5Kf/src/compiler/interface2.jl:0
  [3] _pullback
    @ ~/.julia/dev/ITensors/src/physics/sitetype.jl:187 [inlined]
  [4] _pullback(ctx::Zygote.Context, f::typeof(ITensors._sitetypes), args::TagSet)
    @ Zygote ~/.julia/packages/Zygote/Lw5Kf/src/compiler/interface2.jl:0
  [5] _pullback
    @ ~/.julia/dev/ITensors/src/physics/sitetype.jl:190 [inlined]
  [6] _pullback(ctx::Zygote.Context, f::typeof(ITensors._sitetypes), args::Index{Int64})
    @ Zygote ~/.julia/packages/Zygote/Lw5Kf/src/compiler/interface2.jl:0
  [7] _pullback
    @ ~/.julia/dev/ITensors/src/physics/sitetype.jl:462 [inlined]
  [8] _pullback(::Zygote.Context, ::ITensors.var"##state#927", ::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}, ::typeof(state), ::Index{Int64}, ::String)
    @ Zygote ~/.julia/packages/Zygote/Lw5Kf/src/compiler/interface2.jl:0
  [9] _pullback
    @ ~/.julia/dev/ITensors/src/physics/sitetype.jl:462 [inlined]
 [10] _pullback
...

[mtfishman]

One thing to try to avoid this specific error is define:

@non_differentiable _sitetypes(::Any)

but likely there are other errors too.

[mtfishman]

I just realized that usually I wouldn't expect any variational parameters to be passed to productMPS so maybe it could just be marked with @non_differentiable. But I guess you might initial a product state from some angles.