Fix (deprecated) ONNX exporter to account for new tf2onnx API
# What does this PR do?

This PR fixes an issue with the deprecated `convert_graph_to_onnx` module, where exporting TensorFlow models to ONNX failed due to changes in the `tf2onnx` API. In particular, the failing tests in question were:

* `test_onnx.py::OnnxExportTestCase::test_export_tensorflow`
* `test_onnx.py::OnnxExportTestCase::test_quantize_tf`

With this fix, the whole `test_onnx.py` test suite now passes when I run:

```
RUN_SLOW=1 pytest tests/onnx/test_onnx.py
```
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15856). All of your documentation changes will be reflected on that endpoint.