Feat/onnx models mlflow upload

[AlessandroPolidori]

Summary

Describe the purpose of the pull request, including:

Now mlflow model log is optional (regulated by new core config parameter "upload_models").
Now also onnx and onnx simplified models can be logged on mlflow.
Now matplotlib backend is switched to 'Agg' during analysis report image plotting, to solve major slowdown issue on some machines.
Now also Segmentation prints number of samples of each split (without considering classes, given the one-hot encoding ugly format)

Type of Change

• New feature (non-breaking change that adds functionality)
• Small Bug fix (non-breaking change that solves an issue)

Checklist

Please confirm that the following tasks have been completed:

• I have tested my changes locally and they work as expected. (Please describe the tests you performed.)
• I have added unit tests for my changes, or updated existing tests if necessary.
• I have updated the documentation, if applicable.
• I have installed pre-commit and run locally for my code changes.
  

[lorenzomammana]

Very nice job, just a tiny comment

[lorenzomammana]

LGTM