Add CodeParrot 🦜 codebase
# Add CodeParrot 🦜 codebase

This PR adds the CodeParrot 🦜 codebase to `examples/research_projects/`. The folder `scripts/` includes files for the following steps:

- data preprocessing
- model initialization
- training with accelerate
- validation loss
- evaluate on HumanEval benchmark

In addition the README gives an overview and highlights the results. The requirements file fixes the dependencies.

cc @LysandreJik @thomwolf
> * For each step (preprocessing, training, evaluation etc), I think it would be useful to have a CLI with some default values. Currently, there are some hard-coded variables / configs that require the end-user to read the source code to understand what's going on.

The reason I went without a CLI is that most scripts are quite lean and a CLI would decrease readability. E.g. for initialisation would significantly longer with a CLI. Due to the required compute scale I also expect this to be run by "advanced" users. This reminded me that the training script has to be executed from the accelerate CLI so I'll add a remark about this in any case.

What do you think @LysandreJik? 