Break the exir capture <-> program/verifier import cycle (#24799)

### Summary
`exir/capture/__init__.py` loads `_capture.py`, which imported
`executorch.exir.program` and `executorch.exir.verification.verifier` at
module scope. `verifier.py`, `program/_program.py` and
`_serialize/_serialize.py` all import `executorch.exir.capture._config`,
so each of them ran `capture/__init__.py` and looped back into itself
while still half-initialised.

Normally `exir/__init__.py` loads `capture` first, so the cycle stays
hidden. Under lazy imports (e.g. Cinder lazy imports on Python 3.14) the
first import can enter through one of those modules instead, for example
via `backends/xnnpack/xnnpack_preprocess.py` -> `verifier`, and fails:

```
ImportError: cannot import name 'EXIRATenDialectVerifierBase' from partially initialized module 'executorch.exir.verification.verifier' (most likely due to a circular import)
```

`_capture.py` uses `ExirExportedProgram`, `_transform`,
`HackedUpExportedProgramDONOTUSE` and `EXIRATenDialectVerifierBase` only
inside `capture()` and `_capture_legacy_do_not_use()`, so those imports
move into the two functions, with a `TYPE_CHECKING` import for the
return annotations. No public import paths change.

The separate `exir/backend/backend_api.py` <->
`exir/program/_program.py` cycle is not addressed here.

This PR was authored with AI assistance (Claude Code).

### Test plan
New `exir/tests/test_import_order.py` (collected by OSS pytest, plus a
Buck target) imports `executorch.exir.verification.verifier`,
`executorch.exir.program` and `executorch.exir._serialize._serialize` in
a fresh interpreter whose `executorch.exir` is a bare package, so
`exir/__init__.py` doesn't run first. On main all three fail with a
circular-import error; with this change all three pass. That stub
imports eagerly, so on main it reports a different name than the
lazy-import traceback above (e.g. `EXIRATenDialectVerifier` from
`_program.py`); same cycle.

Smoke-tested `capture()` (with and without `enable_aot`).
`_capture_legacy_do_not_use()` already fails on main with
`ExportedProgram.__init__() got an unexpected keyword argument
'verifier'`, and fails the same way with this change.

`python -m unittest executorch.exir.verification.test.test_verifier
executorch.exir.tests.test_passes
executorch.backends.xnnpack.test.ops.test_add
executorch.backends.xnnpack.test.ops.test_conv2d`: same results as main
(3 errors, `test_to_out_variant_none_output` and two `test_qs8_conv*`,
which also error on main in my environment).

Not yet verified in a real Cinder lazy-imports Python 3.14 runtime.


cc @JacobSzwejbka @angelayi

Author: JakeStevens

exir/capture/_capture.py

@@ -9,21 +9,18 @@
 from collections import namedtuple
 from contextlib import contextmanager
 from types import MethodType
-from typing import Any, Callable, cast, List, Optional, Tuple
+from typing import Any, Callable, cast, List, Optional, Tuple, TYPE_CHECKING
 
 import torch
 from executorch.exir.capture._config import CaptureConfig
 from executorch.exir.error import ExportError, ExportErrorType, InternalError
-from executorch.exir.program import ExirExportedProgram
-from executorch.exir.program._program import _transform, HackedUpExportedProgramDONOTUSE
 from executorch.exir.tracer import (
     _default_decomposition_table,
     dispatch_trace,
     dynamo_trace,
     flatten_output,
     Value,
 )
-from executorch.exir.verification.verifier import EXIRATenDialectVerifierBase
 from torch import _guards
 from torch._dispatch.python import enable_python_dispatcher
 from torch._export.passes import ReplaceViewOpsWithViewCopyOpsPass
@@ -46,6 +43,11 @@
 from torch.fx.experimental.symbolic_shapes import ShapeEnv
 from torch.utils import _pytree as pytree
 
+# exir.program and exir.verification.verifier import executorch.exir.capture
+# (via capture._config), so importing them here at module scope is circular.
+if TYPE_CHECKING:
+    from executorch.exir.program import ExirExportedProgram
+
 
 Val = Any
 
@@ -59,7 +61,7 @@
 
 
 @compatibility(is_backward_compatible=False)
-def _capture_legacy_do_not_use(f, args) -> ExirExportedProgram:
+def _capture_legacy_do_not_use(f, args) -> "ExirExportedProgram":
     """
     This is a legacy API that should be avoided. Prefer to use capture() instead.
     """
@@ -69,6 +71,9 @@ def _capture_legacy_do_not_use(f, args) -> ExirExportedProgram:
         DeprecationWarning,
         stacklevel=1,
     )
+    from executorch.exir.program import ExirExportedProgram
+    from executorch.exir.program._program import HackedUpExportedProgramDONOTUSE
+    from executorch.exir.verification.verifier import EXIRATenDialectVerifierBase
 
     graph_module = dispatch_trace(f, args)
     flat_args = tuple(pytree.tree_flatten(args)[0])
@@ -167,12 +172,16 @@ def capture(  # noqa: C901
     args: Tuple[Value, ...],
     config: Optional[CaptureConfig] = None,
     dynamic_shapes: Optional[List[Any]] = None,
-) -> ExirExportedProgram:
+) -> "ExirExportedProgram":
     warnings.warn(
         "This function is now deprecated, please use `torch.export and exir.to_edge` instead. ",
         DeprecationWarning,
         stacklevel=1,
     )
+    from executorch.exir.program import ExirExportedProgram
+    from executorch.exir.program._program import _transform
+    from executorch.exir.verification.verifier import EXIRATenDialectVerifierBase
+
     if not isinstance(args, tuple):
         raise ExportError(
             ExportErrorType.INVALID_INPUT_TYPE,

exir/tests/targets.bzl

@@ -114,6 +114,16 @@ def define_common_targets(is_fbcode = False):
         ],
     )
 
+    python_unittest(
+        name = "import_order",
+        srcs = [
+            "test_import_order.py",
+        ],
+        deps = [
+            "//executorch/exir:lib",
+        ],
+    )
+
     python_unittest(
         name = "tensor",
         srcs = [

exir/tests/test_import_order.py

@@ -0,0 +1,39 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+# pyre-strict
+import subprocess
+import sys
+import unittest
+
+_IMPORT_WITHOUT_EXIR_INIT = """
+import importlib, importlib.util, sys, types
+
+spec = importlib.util.find_spec("executorch.exir")
+exir = types.ModuleType("executorch.exir")
+exir.__path__ = spec.submodule_search_locations
+sys.modules["executorch.exir"] = exir
+importlib.import_module(sys.argv[1])
+"""
+
+
+class TestImportOrder(unittest.TestCase):
+    def test_import_without_exir_init(self) -> None:
+        # Lazy imports can defer executorch/exir/__init__.py, so each of these
+        # must import cleanly as the first executorch.exir module.
+        for module in (
+            "executorch.exir.verification.verifier",
+            "executorch.exir.program",
+            "executorch.exir._serialize._serialize",
+        ):
+            with self.subTest(module=module):
+                completed = subprocess.run(
+                    [sys.executable, "-c", _IMPORT_WITHOUT_EXIR_INIT, module],
+                    capture_output=True,
+                    text=True,
+                    timeout=300,
+                )
+                self.assertEqual(completed.returncode, 0, completed.stderr)