Inserting a known derivative: function needs to exclude dual numbers

[kimauth]

I ran into the following problem today:

function tensor_exp(A::SymmetricTensor{2})
    E = eigen(A)
    A_exp = zero(A)
    for (i, λ) in enumerate(E.values)
        N = E.vectors[:,i]
        A_exp += exp(λ) * N ⊗ N
    end
    return A_exp
end

tensor_exp_gradient(A::AbstractTensor{2}) = tensor_exp(A), tensor_exp(A)

@implement_gradient tensor_exp tensor_exp_gradient

julia> A = rand(SymmetricTensor{2,3}) + one(SymmetricTensor{2,3})
julia> gradient(tensor_exp, A)

ERROR: MethodError: no method matching precision(::Type{ForwardDiff.Dual{ForwardDiff.Tag{typeof(tensor_exp), SymmetricTensor{2, 3, Float64, 6}}, Float64, 6}})
Closest candidates are:
  precision(::Type{Float16}) at C:\Users\auth\AppData\Local\Programs\Julia-1.7.2\share\julia\base\float.jl:686
  precision(::Type{Float32}) at C:\Users\auth\AppData\Local\Programs\Julia-1.7.2\share\julia\base\float.jl:687
  precision(::Type{Float64}) at C:\Users\auth\AppData\Local\Programs\Julia-1.7.2\share\julia\base\float.jl:688
  ...
Stacktrace:
 [1] eigen(R::SymmetricTensor{2, 3, ForwardDiff.Dual{ForwardDiff.Tag{typeof(tensor_exp), SymmetricTensor{2, 3, Float64, 6}}, Float64, 6}, 6})
   @ Tensors C:\Users\auth\.julia\packages\Tensors\fjqpn\src\eigen.jl:137
 [2] tensor_exp(A::SymmetricTensor{2, 3, ForwardDiff.Dual{ForwardDiff.Tag{typeof(tensor_exp), SymmetricTensor{2, 3, Float64, 6}}, Float64, 6}, 6})
   @ Main c:\Users\auth\OneDrive - Chalmers\Documents\courses\Phd courses\Computational nonlinear mechanics\computer assignments\cass3\tensor_log_exp.jl:29
 [3] gradient(f::typeof(tensor_exp), v::SymmetricTensor{2, 3, Float64, 6})
   @ Tensors C:\Users\auth\.julia\packages\Tensors\fjqpn\src\automatic_differentiation.jl:455
 [4] top-level scope
   @ REPL[6]:1

The problem here is that we run into the tensor_exp function with dual numbers (instead of using tensor_exp_gradient).
Looking at the methods of tensor_exp, we can see why:

julia> methods(tensor_exp)
# 2 methods for generic function "tensor_exp":
[1] tensor_exp(A::SymmetricTensor{2}) in Main at c:\Users\auth\OneDrive - Chalmers\Documents\courses\Phd courses\Computational nonlinear mechanics\computer assignments\cass3\tensor_log_exp.jl:28
[2] tensor_exp(x::Union{ForwardDiff.Dual, AbstractTensor{<:Any, <:Any, <:ForwardDiff.Dual}}) in Main at C:\Users\auth\.julia\packages\Tensors\fjqpn\src\automatic_differentiation.jl:253

The original tensor_exp is more specific than the one defined for Dual numbers by @implement_gradient. The solution could be to not allow dual numbers in the original function at all, e.g. by

function tensor_exp(A::SymmetricTensor{2,dim,Float64}) where dim

(This is of course not so nice if one doesn't own this function. )

Perhaps there is a better solution than specifying the number type of the Tensor. In case there isn't we should probably add a hint about it to the docs.

[KnutAM]

One solution is to document and export the _propagate_gradient function used by @implement_gradient. It is not as sleek as the macro but allows the user to solve such a problem. I.e.

tensor_exp(A::SymmetricTensor{2,dim,<:ForwardDiff.Dual}) where{dim} = Tensors._propagate_gradient(tensor_exp_gradient, A)

For this to work the output of tensor_exp must be a symmetric tensor, i.e.

function tensor_exp(A::SymmetricTensor{2})
    E = eigen(A)
    A_exp = zero(A)
    for (i, λ) in enumerate(E.values)
        N = E.vectors[:,i]
        A_exp += exp(λ) * otimes(N)
    end
    return A_exp
end