Track code version with model version

[sandhawalia]

Currently we don't track code version with the model version this can lead to inability to re-produce results.

• Use git python package to get commit tag.
• Add git commit tag as a prefix when saving model files
• Add git commit tag to logs
• Add git commit tags to yaml config files at exp_dir

[sandhawalia]

Addressed here https://github.com/moabitcoin/cherry-pytorch/pull/32

[sandhawalia]

The models are saved as agent-{step}-{gitsha}.pth