Fix PytorchModel when last layer doesn't support out_features (#166)

ext/MathOptAIPythonCallExt.jl

@@ -151,9 +151,14 @@ function MathOptAI.GrayBox(
     torch_model = torch_model.to(device)
     J = torch.func.jacrev(torch_model)
     H = torch.func.hessian(torch_model)
-    # TODO(odow): I'm not sure if there is a better way to get the output
-    # dimension of a torch model object?
-    output_size(::Any) = PythonCall.pyconvert(Int, torch_model[-1].out_features)
+    function output_size(x::Vector)
+        # Get the output size by passing a zero vector through the torch model.
+        # We do this instead of `torch_model[-1].out_features` as the last layer
+        # may not support out_features.
+        z = torch.zeros(length(x))
+        y = torch_model(z)
+        return PythonCall.pyconvert(Int, PythonCall.pybuiltins.len(y))
+    end
     function callback(x)
         py_x = torch.tensor(collect(x); device = device)
         py_value = torch_model(py_x).detach().cpu().numpy()

test/test_PythonCall.jl

@@ -453,6 +453,55 @@ function test_model_Tanh_vector_GrayBox_hessian()
     return
 end
 
+function test_model_Sigmoid_last_layer_GrayBox()
+    dir = mktempdir()
+    filename = joinpath(dir, "model_Sigmoid_last_layer_GrayBox.pt")
+    PythonCall.pyexec(
+        """
+        import torch
+
+        model = torch.nn.Sequential(
+            torch.nn.Linear(3, 16),
+            torch.nn.Sigmoid(),
+        )
+
+        torch.save(model, filename)
+        """,
+        @__MODULE__,
+        (; filename = filename),
+    )
+    # Full-space
+    model = Model(Ipopt.Optimizer)
+    set_silent(model)
+    @variable(model, x[i in 1:3] == i)
+    ml_model = MathOptAI.PytorchModel(filename)
+    y, formulation =
+        MathOptAI.add_predictor(model, ml_model, x; gray_box = true)
+    @test num_variables(model) == 19
+    @test num_constraints(model; count_variable_in_set_constraints = true) == 19
+    optimize!(model)
+    @test is_solved_and_feasible(model)
+    @test ≈(_evaluate_model(filename, value.(x)), value.(y); atol = 1e-5)
+    # Reduced-space
+    model = Model(Ipopt.Optimizer)
+    set_silent(model)
+    @variable(model, x[i in 1:3] == i)
+    ml_model = MathOptAI.PytorchModel(filename)
+    y, formulation = MathOptAI.add_predictor(
+        model,
+        ml_model,
+        x;
+        gray_box = true,
+        reduced_space = true,
+    )
+    @test num_variables(model) == 3
+    @test num_constraints(model; count_variable_in_set_constraints = true) == 3
+    optimize!(model)
+    @test is_solved_and_feasible(model)
+    @test ≈(_evaluate_model(filename, value.(x)), value.(y); atol = 1e-5)
+    return
+end
+
 end  # module
 
 TestPythonCallExt.runtests()