Fix classification test loop crashing (due to non-existing directory) + updates in arrays utils and the overall training script

[joschkabirk]

Summary

This pull request introduces the following changes

• Fix directory not existing at end of test loop in classification callback
• Update classification example job script
• Some updates in utils in arrays.py
• Updates in main training script (train.py) for better handling of checkpoints
• Needed updates for multi-gpu