Mrbean/codegen onnx
# What does this PR do?
Codegen was added with an ONNX config but not with the model added to the features manager so trying to actually export an ONNX config is failing.

```bash
11497 ± RUN_SLOW=1 pytest tests/onnx/test_onnx_v2.py -k "codegen" -v
===================================================================================== test session starts ======================================================================================
platform darwin -- Python 3.9.10, pytest-7.1.2, pluggy-1.0.0 -- /Users/marklar/workspace/transformers/venv/bin/python3
cachedir: .pytest_cache
hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase('/Users/marklar/workspace/transformers/.hypothesis/examples')
rootdir: /Users/marklar/workspace/transformers, configfile: setup.cfg
plugins: xdist-2.5.0, forked-1.4.0, timeout-2.1.0, hypothesis-6.47.0, dash-2.5.0
collected 371 items / 367 deselected / 4 selected                                                                                                                                              

tests/onnx/test_onnx_v2.py::OnnxExportTestCaseV2::test_pytorch_export_029_codegen_causal_lm PASSED                                                                                       [ 25%]
tests/onnx/test_onnx_v2.py::OnnxExportTestCaseV2::test_pytorch_export_030_codegen_default PASSED                                                                                         [ 50%]
tests/onnx/test_onnx_v2.py::OnnxExportTestCaseV2::test_pytorch_export_on_cuda_029_codegen_causal_lm PASSED                                                                               [ 75%]
tests/onnx/test_onnx_v2.py::OnnxExportTestCaseV2::test_pytorch_export_on_cuda_030_codegen_default PASSED                                                                                 [100%]

```


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- [x] Did you write any new necessary tests?


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@patil-suraj @JingyaHuang 
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