Adding unit test for prepare_model and minor bug fixes

Summary

This MR introduces the following changes

• Adding a unit test for the prepare_model function.
• Adding a -t option to xargs to print all the commands executed.
• Moving a logger.warning to a debug which prints the plot_args.
• Adding trained_tagger to excluded variable which are not added to plot_args.

Conformity

• Changelog entry
• Documentation
• Development guidelines
• Style guides