[IR] Implement register_initializer (#1941)

Implement `register_initializer(value)` on `Graph` for robustly adding
an initializer to the graph. Users can also directly modify the
`graph.initializers` dictionary, but this method does more comprehensive
checks before adding, and calling this method is simpler than modifying
the dictionary.

The method could be in the `convenience` too, but I put it here due to
its relevance and for better discoverability.

onnxscript/ir/_core.py

@@ -1824,6 +1824,42 @@ def outputs(self) -> list[Value]:
     def initializers(self) -> dict[str, Value]:
         return self._initializers
 
+    def register_initializer(self, value: Value) -> None:
+        """Register an initializer to the graph.
+
+        This is a convenience method to register an initializer to the graph with
+        checks.
+
+        Args:
+            value: The :class:`Value` to register as an initializer of the graph.
+                It must have its ``.const_value`` set.
+
+        Raises:
+            ValueError: If a value of the same name that is not this value
+                is already registered.
+            ValueError: If the value does not have a name.
+            ValueError: If the initializer is produced by a node.
+            ValueError: If the value does not have its ``.const_value`` set.
+        """
+        if value.name in self._initializers:
+            if self._initializers[value.name] is not value:
+                raise ValueError(
+                    f"Initializer '{value.name}' is already registered, but"
+                    " it is not the same object: existing={self._initializers[value.name]!r},"
+                    f" new={value!r}"
+                )
+        if not value.name:
+            raise ValueError(f"Initializer must have a name: {value!r}")
+        if value.producer() is not None:
+            raise ValueError(
+                f"Value '{value!r}' is produced by a node and cannot be an initializer."
+            )
+        if value.const_value is None:
+            raise ValueError(
+                f"Value '{value!r}' must have its const_value set to be an initializer."
+            )
+        self._initializers[value.name] = value
+
     @property
     def doc_string(self) -> str | None:
         return self._doc_string

onnxscript/ir/_core_test.py

@@ -843,6 +843,30 @@ def test_remove_safe_removes_uses_of_removed_nodes(self):
         self.assertEqual(tuple(graph), (sub_node, identity_node))
         self.assertEqual(add_node.inputs, (None, None))
 
+    def test_register_initializer(self):
+        self.v1.const_value = ir.tensor([1, 2, 3])
+        self.graph.register_initializer(self.v1)
+        self.assertEqual(self.graph.initializers, {self.v1.name: self.v1})
+
+    def test_register_initializer_raises_when_value_is_not_constant(self):
+        with self.assertRaises(ValueError):
+            self.graph.register_initializer(self.v0)
+
+    def test_register_initializer_raises_when_a_different_value_is_already_registered(self):
+        self.v1.const_value = ir.tensor([1, 2, 3])
+        self.graph.register_initializer(self.v1)
+        # This is fine
+        self.graph.register_initializer(self.v1)
+        self.v0.name = "v1"
+        with self.assertRaisesRegex(ValueError, "already registered"):
+            # Registering a different value with the same name should raise
+            self.graph.register_initializer(self.v0)
+
+    def test_register_initializer_raises_when_value_does_not_have_a_name(self):
+        self.v1.name = None
+        with self.assertRaises(ValueError):
+            self.graph.register_initializer(self.v1)
+
     # TODO(justinchuby): Test graph mutation methods
 
     # Test topological sort.