Support OptionalType to get shape | feat(graph_building) (#890)

Due to the need of supporting None args, we need to support None
(torch.OptionalType) to get shape/dtype.
To support pytorch/pytorch#105263

---------

Co-authored-by: Justin Chu <justinchuby@users.noreply.github.com>

onnxscript/function_libs/torch_lib/graph_building.py

@@ -129,7 +129,10 @@ def shape(self) -> Tuple[int | None, ...] | None:
         if value_type is None:
             return None
         value_type = typing.cast(torch.TensorType, value_type)
-        shape = value_type.varyingSizes()
+        if isinstance(value_type, torch.OptionalType):
+            shape = value_type.getElementType().varyingSizes()  # type: ignore[attr-defined]
+        else:
+            shape = value_type.varyingSizes()
         if shape is None:
             return None
         return tuple(shape)

onnxscript/function_libs/torch_lib/graph_building_test.py

@@ -118,6 +118,12 @@ def outer(x: FLOAT[1, 2, 3]):
         expected = outer.to_model_proto()
         onnxscript.testing.assert_isomorphic(traced, expected)
 
+    def test_add_input_with_optionaltype_does_not_raise_torch_internal_error(self):
+        graph = graph_building.TorchScriptGraph()
+        x = graph.add_input(input_name=None)
+        with evaluator.default_as(self.tracer):
+            _ = x.shape
+
 
 class TestTorchScriptGraph(unittest.TestCase):
     def test_add_initializer_raises_when_the_same_name_used_for_different_tensors(self):