Move _c_api_utils.py to version_converter package (#2401)

This PR moves `_c_api_utils.py` from `onnxscript/ir/passes/common/` to
`onnxscript/version_converter/` since it's only used by the version
converter module.

## Changes
- Moved `onnxscript/ir/passes/common/_c_api_utils.py` to
`onnxscript/version_converter/_c_api_utils.py`
- Updated import in `onnxscript/version_converter/__init__.py` from
`from onnxscript.ir.passes.common import _c_api_utils` to `from . import
_c_api_utils`

## Analysis
A codebase analysis confirmed that `_c_api_utils.py` is only imported
and used by the version converter:
- The file contains utilities for interfacing with ONNX C APIs,
specifically the `call_onnx_api` function
- It's only imported in `onnxscript/version_converter/__init__.py` 
- It's not exported in any `__all__` lists
- No other modules reference or use this utility

Moving the file to the version converter package improves code
organization by colocating the utility with its sole consumer.

Fixes #2400.

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: justinchuby <[email protected]>
Co-authored-by: Justin Chu <[email protected]>

Author: Copilot

onnxscript/version_converter/__init__.py

@@ -14,8 +14,7 @@
 import onnxscript.ir.passes
 import onnxscript.ir.passes.common
 from onnxscript import ir
-from onnxscript.ir.passes.common import _c_api_utils
-from onnxscript.version_converter import _version_converter
+from onnxscript.version_converter import _c_api_utils, _version_converter
 
 logger = logging.getLogger(__name__)