New features for CodeParrot training script
This PR adds some features to CodeParrot training script.

- Add TFLOPS to logging
- Use Accelerate checkpointing and tracking for Wandb and Tensorborad
- Fix gradient accumulation for DDP (https://github.com/huggingface/accelerate/pull/106)
- Scale loss approprietly for the last batch
- Fix typo in the README

cc @lvwerra @LysandreJik 
_The documentation is not available anymore as the PR was closed or merged._