WIP: disable gradient and Hessian backends for NLSModels

A user can still enable them manually if they really want to.

Author: dpo

src/ad.jl

@@ -218,14 +218,14 @@ function ADModelNLSBackend(
   backend::Symbol = :default,
   matrix_free::Bool = false,
   show_time::Bool = false,
-  gradient_backend = get_default_backend(:gradient_backend, backend),
-  hprod_backend = get_default_backend(:hprod_backend, backend),
-  hessian_backend = get_default_backend(:hessian_backend, backend, matrix_free),
-  hprod_residual_backend = get_default_backend(:hprod_residual_backend, backend),
+  gradient_backend = EmptyADbackend(),
+  hprod_backend = EmptyADbackend(),
+  hessian_backend = EmptyADbackend(),
+  hprod_residual_backend = EmptyADbackend(),
   jprod_residual_backend = get_default_backend(:jprod_residual_backend, backend),
   jtprod_residual_backend = get_default_backend(:jtprod_residual_backend, backend),
   jacobian_residual_backend = get_default_backend(:jacobian_residual_backend, backend, matrix_free),
-  hessian_residual_backend = get_default_backend(:hessian_residual_backend, backend, matrix_free),
+  hessian_residual_backend = EmptyADbackend(),
   kwargs...,
 )
   function F(x; nequ = nequ)
@@ -343,18 +343,18 @@ function ADModelNLSBackend(
   backend::Symbol = :default,
   matrix_free::Bool = false,
   show_time::Bool = false,
-  gradient_backend = get_default_backend(:gradient_backend, backend),
-  hprod_backend = get_default_backend(:hprod_backend, backend),
+  gradient_backend = EmptyADbackend(),
+  hprod_backend = EmptyADbackend(),
   jprod_backend = get_default_backend(:jprod_backend, backend),
   jtprod_backend = get_default_backend(:jtprod_backend, backend),
   jacobian_backend = get_default_backend(:jacobian_backend, backend, matrix_free),
-  hessian_backend = get_default_backend(:hessian_backend, backend, matrix_free),
-  ghjvprod_backend = get_default_backend(:ghjvprod_backend, backend),
-  hprod_residual_backend = get_default_backend(:hprod_residual_backend, backend),
+  hessian_backend = EmptyADbackend(),
+  ghjvprod_backend = EmptyADbackend(),
+  hprod_residual_backend = EmptyADbackend(),
   jprod_residual_backend = get_default_backend(:jprod_residual_backend, backend),
   jtprod_residual_backend = get_default_backend(:jtprod_residual_backend, backend),
   jacobian_residual_backend = get_default_backend(:jacobian_residual_backend, backend, matrix_free),
-  hessian_residual_backend = get_default_backend(:hessian_residual_backend, backend, matrix_free),
+  hessian_residual_backend = EmptyADbackend(),
   kwargs...,
 )
   function F(x; nequ = nequ)