CodeParrot data pretokenization
This PR adds code for data pretokenization of CodeParrot . In fact it takes long to tokenize the data especially for small models, so having a pretokenized dataset might improve the training speed. We also fix an error in the `README.md` and `scripts/initialize_model.py` inside the codeparrot repo. 

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