Fix exporting issue (#239)

Summary:
Pull Request resolved: #239

While Jarvis' CI was down for another issue, we need to propagate changes from D48917505 to the fix introduced concurrently in D48838464.

Reviewed By: angelayi

Differential Revision: D49070726

fbshipit-source-id: 55761c0237da8b142016e4cf3eea95929db42387

Author: mcremon-meta

exir/capture/_capture.py

@@ -65,10 +65,17 @@ def _capture_legacy_do_not_use(f, args) -> ExirExportedProgram:
     _instantiate_missing_placeholder_val_with_real_inputs(graph_module, flat_args)
     graph_module._apply(torch.Tensor.contiguous)
 
+    user_inputs = [
+        node.name for node in graph_module.graph.nodes if node.op == "placeholder"
+    ]
+    output_node = list(graph_module.graph.nodes)[-1]
+    assert output_node.op == "output"
+    user_outputs = [arg.name for arg in output_node.args[0]]
+
     ep = HackedUpExportedProgramDONOTUSE(
         graph_module,
         graph_module.graph,
-        ExportGraphSignature([], [], [], [], {}, {}, {}, None),
+        ExportGraphSignature([], [], user_inputs, user_outputs, {}, {}, {}, None),
         CallSpec(in_spec, out_spec),
         {},
         {},