[API] Create stable APIs for PyTorch 2.5 (#1832)

Create stable APIs for PyTorch 2.5 so that it does not need to use any
internal ONNX Script APIs. Created APIs are

```
"check_model",
"convert_version",
"get_torchlib_ops",
"optimize",
"save_model_with_external_data",
```

In pytorch, it is expected to write:

```python
import onnxscript._framework_apis.torch_2_5
```

Fixes #1827

onnxscript/_framework_apis/__init__.py

@@ -0,0 +1,3 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+"""Semi-private stable APIs for framework-specific usage only."""

onnxscript/_framework_apis/torch_2_5.py

@@ -0,0 +1,160 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+"""Stable APIs for PyTorch 2.5."""
+
+from __future__ import annotations
+
+__all__ = [
+    "check_model",
+    "convert_version",
+    "get_torchlib_ops",
+    "optimize",
+    "save_model_with_external_data",
+]
+
+import dataclasses
+import os
+import pathlib
+from typing import Callable
+
+import onnx
+
+from onnxscript import ir
+from onnxscript.function_libs.torch_lib import registration
+from onnxscript.ir import _external_data
+
+# Internal flag. Will go away.
+_TORCH_ONNX_SAVE_EXTERNAL_DATA_WITH_IR = (
+    os.getenv("TORCH_ONNX_OFFLOAD_EXTERNAL_DATA_WITH_IR") == "1"
+)
+
+
+@dataclasses.dataclass(frozen=True)
+class _OnnxFunctionMeta:
+    """A wrapper of onnx-script function with additional metadata.
+
+    qualified_name: The qualified name of the aten operator.
+    function: The onnx-script function.
+    domain: The domain of the function.
+    name: The name of the function.
+    is_complex: Whether the function is a complex function.
+    """
+
+    qualified_name: str
+    function: Callable
+    domain: str
+    name: str
+    is_complex: bool = False
+
+
+def optimize(model: ir.Model) -> ir.Model:
+    """Optimize the model."""
+
+    # TODO(justinchuby): Use the optimizer
+    return model
+
+
+def convert_version(model: ir.Model, target_version: int) -> ir.Model:
+    """Convert the model to the specified ONNX opset version."""
+    # model_version = model.opset_import.get("")
+    # if model_version == target_version:
+    #     # No conversion needed
+    #     return model
+
+    # # FIXME(justinchuby): version_converter does not support functions
+    # proto = ir.serde.serialize_model(model)
+    # proto = onnx.version_converter.convert_version(proto, target_version)
+    # return ir.serde.deserialize_model(proto)
+    # TODO(justinchuby): This function needs to be carefully implemented
+    # to handle large models. For now, we just return the model.
+    del target_version  # Unused
+    return model
+
+
+def check_model(model: ir.Model) -> None:
+    """Check the model."""
+
+    del model  # Unused yet
+
+
+def save_model_with_external_data(model: ir.Model, model_path: str | os.PathLike) -> None:
+    """Save the model with external data. The model is unchanged after saving."""
+
+    # TODO(#1835): Decide if we want to externalize large attributes as well
+    if _TORCH_ONNX_SAVE_EXTERNAL_DATA_WITH_IR:
+        initializer_values = tuple(model.graph.initializers.values())
+        tensors = [v.const_value for v in initializer_values]
+        for tensor in tensors:
+            if tensor is None:
+                raise ValueError(
+                    "The model contains uninitialized initializer values. "
+                    "Please make sure all initializer values are initialized."
+                )
+        destination_path = pathlib.Path(model_path)
+        base_dir = destination_path.parent
+        data_path = f"{destination_path.name}.data"
+
+        external_tensors = _external_data.convert_tensors_to_external(
+            tensors,  # type: ignore[arg-type]
+            base_dir,
+            data_path,
+        )
+
+        # Replace the initializer values with external tensors and save the model
+        for initializer, external_tensor in zip(initializer_values, external_tensors):
+            initializer.const_value = external_tensor
+        ir.save(model, model_path)
+
+        # Restore the original initializer values so the model is unchanged
+        for initializer, tensor in zip(initializer_values, tensors):
+            initializer.const_value = tensor
+
+    else:
+        destination_path = pathlib.Path(model_path)
+        # Create the directory if it does not exist
+        data_path = f"{destination_path.name}.data"
+        proto = ir.serde.serialize_model(model)
+        onnx.save_model(
+            proto,
+            model_path,
+            save_as_external_data=True,
+            location=data_path,
+        )
+
+
+def get_torchlib_ops() -> list[_OnnxFunctionMeta]:
+    # Trigger op registration
+    from onnxscript.function_libs.torch_lib import (  # pylint: disable=import-outside-toplevel
+        ops,
+    )
+
+    del ops  # Unused
+
+    torchlib_registry = registration.default_registry
+    function_metas = []
+
+    for qualified_name, aten_overloads_func in torchlib_registry.items():
+        if qualified_name.startswith("internal::"):
+            # Skip the custom defined internal functions
+            continue
+
+        for overload_func in aten_overloads_func.overloads:
+            function_meta = _OnnxFunctionMeta(
+                qualified_name=qualified_name,
+                function=overload_func,
+                domain=overload_func.function_ir.domain,
+                name=overload_func.name,
+                is_complex=False,
+            )
+            function_metas.append(function_meta)
+        for complex_func in aten_overloads_func.complex:
+            function_meta = _OnnxFunctionMeta(
+                qualified_name=qualified_name,
+                function=complex_func,
+                domain=complex_func.function_ir.domain,
+                name=complex_func.name,
+                is_complex=True,
+            )
+            function_metas.append(function_meta)
+
+    return function_metas

pyproject.toml

@@ -210,7 +210,7 @@ ignore-init-module-imports = true
 [tool.ruff.lint.per-file-ignores]
 "__init__.py" = ["TID252"] # Allow relative imports in init files
 "setup.py" = ["TID251"] # pathlib is allowed in supporting code
-"**/{examples,tests,docs,tools,utils,opgen}/*" = ["TID251"] # pathlib is allowed in supporting code
+"**/{examples,tests,docs,tools,utils,opgen,_framework_apis}/*" = ["TID251"] # pathlib is allowed in supporting code
 "**/*_test.py" = ["TID251"] # pathlib is allowed in tests
 
 [tool.ruff.lint.flake8-tidy-imports]