Onnx bibentry structure

[cmwilhelm]

Provides onnx support for bibentry detector.

The new artifact referenced in the timo config yaml was produced via:

tt fetch-artifacts -c ai2_internal/config.yaml -a bibentry_detector_artifacts
docker run -v <currdir>/bibentry_detector_artifacts:/opt/ml/model:rw --rm -it --entrypoint /bin/bash python:3.8

# in docker container
pip install transformers[onnx]
pip install torch
python -m transformers.onnx --model=/opt/ml/model --feature=token-classification --framework pt --preprocessor tokenizer --opset 16 /opt/ml/model