Add nonlinear_model function

Author: blegat

src/Nonlinear/model.jl

@@ -77,6 +77,16 @@ function set_objective(model::Model, ::Nothing)
     return
 end
 
+"""
+    nonlinear_model(backend::AbstractAutomaticDifferentiation)
+
+Return a new nonlinear model appropriate for the given AD `backend`.
+
+The default returns `Model()`. Custom AD backends can override this
+to return their own model type.
+"""
+nonlinear_model(::AbstractAutomaticDifferentiation) = Model()
+
 """
     add_expression(model::Model, expr)::ExpressionIndex