Add tests for training code

.gitattributes

.gitlab-ci.yml

dan/manager/dataset.py

dan/manager/ocr.py

dan/manager/training.py

dan/ocr/document/train.py

dan/ocr/line/generate_synthetic.py

dan/ocr/line/train.py

tests/__init__.py

tests/conftest.py

tests/data/evaluation/predict_training-test_0.txt

tests/data/evaluation/predict_training-train_0.txt

tests/data/evaluation/predict_training-val_0.txt

tests/data/training/best_0.pt

tests/data/training/last_3.pt

tests/data/training_page/charset.pkl

...ning_page/images/0a34e13a-4ab0-4a91-8d7c-b1d8fee32628.png

...ning_page/images/0a576062-303c-4893-a729-c09c92865d31.png

...ning_page/images/0a70e14f-feda-4607-989c-36cf581ddff5.png

...ning_page/images/0b2457c8-81f1-4600-84d9-f8bf2822a991.png