fix failing conversion notebooks

[cwmeijer]

Notebooks .\tutorials\conversion_onnx\tensorflow2onnx.ipynb and .\tutorials\conversion_onnx\keras2onnx.ipynb fail on the main branch. See https://github.com/dianna-ai/dianna/actions/runs/5609040265/jobs/10262215736. They pass just fine on my machine. I don't know what is causing them to fail on the server. The problem is with finding some model file that was outputted earlier in the notebook. Maybe the file location or different working directory on the server has something to do with it.

[cwmeijer]

accidentally closed.

[cwmeijer]

The problem is that on the server, it cannot find some onnx model files that are generated within earlier cells of the notebooks. I ran the tests locally from the dianna package root folder and from its parent folder. Both without issues. No clue what could be the issue on the server.