Clear initializers in constant folding pass (#2668)

Clear unused initializers on the fly to prevent memory usage jump due to
intermediate folded tensors.

---------

Signed-off-by: Justin Chu <justinchuby@users.noreply.github.com>

Author: justinchuby

VERSION

@@ -1 +1 @@
-0.5.5
+0.5.6

onnxscript/optimizer/_constant_folding.py

@@ -1256,10 +1256,20 @@ def replace_node(
 
         # Record the names of the values that has contributed to the replacement
         _record_contributing_values(node, replacement)
+
+        # Obtain the list of non-None inputs to the node before it is cleared by
+        # replace_nodes_and_values to check for unused initializers later.
+        node_inputs = [v for v in node.inputs if v is not None]
+
         ir.convenience.replace_nodes_and_values(
             root, node, [node], replacement.new_nodes, node.outputs, replacement.new_outputs
         )
 
+        if isinstance(root, ir.Graph):
+            # The old node should now be detached from the graph
+            assert node.graph is None
+            _clear_unused_initializers(node_inputs)
+
         self._modified = True
 
         # TODO: what about new opset_imports?
@@ -1336,6 +1346,19 @@ def _sym_value_can_replace_graph_output(
     return True
 
 
+def _clear_unused_initializers(values: Sequence[ir.Value]) -> None:
+    # Detach all inputs to the node, then check for unused initializers
+    for value in values:
+        if value is None or not value.is_initializer():
+            continue
+
+        if not value.uses():
+            assert value.is_initializer()
+            assert value.graph is not None
+            assert value.name is not None
+            value.graph.initializers.pop(value.name)
+
+
 @dataclasses.dataclass
 class FoldConstantsResult(ir.passes.PassResult):
     symbolic_value_map: dict[ir.Value, SymbolicValue]

onnxscript/optimizer/_constant_folding_test.py

@@ -14,13 +14,20 @@
 
 
 class FoldConstantsTest(unittest.TestCase):
-    def _fold(self, model: ir.Model | str, onnx_shape_inference=False, **kwargs):
+    def _fold(
+        self,
+        model: ir.Model | str,
+        onnx_shape_inference: bool = False,
+        dce: bool = True,
+        **kwargs,
+    ):
         if isinstance(model, str):
             model = ir.from_onnx_text(model)
         _constant_folding.fold_constants(
             model, onnx_shape_inference=onnx_shape_inference, **kwargs
         )
-        optimizer.remove_unused_nodes(model)
+        if dce:
+            optimizer.remove_unused_nodes(model)
         # Ensure the model is valid after optimization
         onnx.checker.check_model(ir.serde.serialize_model(model))
         return model
@@ -50,9 +57,16 @@ def test_fold_cast_like(self):
             }
         """
 
-        optimized = self._fold(model)
-        self.assertEqual(len(optimized.graph), 1)
+        optimized = self._fold(model, dce=False)
         self.assertIn("four", optimized.graph.initializers)
+        np.testing.assert_equal(
+            optimized.graph.initializers["four"].const_value, np.array(4.0)
+        )
+        # Intermediates should be removed
+        self.assertNotIn("two_float", optimized.graph.initializers)
+
+        optimized = self._fold(model, dce=True)
+        self.assertEqual(len(optimized.graph), 1)
 
     def test_fold_shape(self):
         model = """
@@ -66,9 +80,18 @@ def test_fold_shape(self):
             }
         """
 
-        optimized = self._fold(model)
-        self.assertEqual(len(optimized.graph), 1)
+        optimized = self._fold(model, dce=False)
         self.assertIn("four", optimized.graph.initializers)
+        np.testing.assert_equal(
+            optimized.graph.initializers["four"].const_value, np.array(4.0)
+        )
+        # Intermediates should be removed
+        self.assertNotIn("two_float", optimized.graph.initializers)
+        self.assertNotIn("rank", optimized.graph.initializers)
+        self.assertNotIn("shape", optimized.graph.initializers)
+
+        optimized = self._fold(model, dce=True)
+        self.assertEqual(len(optimized.graph), 1)
 
     def test_fold_shape_slice(self):
         model = """