Add Mlflow tracking

[andodet]

Summary

As mentioned in #17, adding training run and experiment tracking capabilities using Mlflow for the following templates:

• templates/Image classification_PyTorch
• templates/Image classification_scikit-learn

Details

This PR implements basic metric/model info logging features. As a second logical next step, model artifact logging (checkpoints and final model) will be added.

Checklist

• [x] all tests are passing (see README.md on how to run tests)
• [ ] if you created a new template: it contains a file test-inputs.yml, which specifies a few input values to test the code template (the test is then automatically run by pytest)
• [x] you formatted all code with black
• [x] you checked all new functionality live, i.e. in the running web app
• [x] any generated code is formatted nicely, both in .py and in .ipynb ("nicely" = comparable to the existing templates)
• [ ] you added comments in your code that explain what it does
• [x] the PR explains in detail what's new