Add CrystalCoder Model
Author: TianhuaTao
Pull request: yes

# What does this PR do?

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This is the implementation for [CrystalCoder](https://huggingface.co/LLM360/CrystalCoder) and [CrystalChat](https://huggingface.co/LLM360/CrystalChat) model by LLM360. 

There is an additional example script on running the model at `src/transformers/models/crystalcoder/crystalchat_example.py`

## Before submitting
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- text models: @ArthurZucker and @younesbelkada
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- trainer: @muellerzr and @pacman100

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Comments:
Hi @TianhuaTao, thanks for opening this PR! 

The easiest and recommended way to make a model available in `transformers` is to add the modeling code directly on the hub: https://huggingface.co/docs/transformers/custom_models. We have as much support there as we can (let us know if anything isn't working 🤗 !)

This means, once working, the model can be found and used immediately without having to go through the PR process. We find this is a lot quicker as the bar for adding code into the library is high due to the maintenance cost of every new model, and so reviews take quite a while.

Hi @amyeroberts , thanks for your prompt reply! 
We already implemented CrystalCoder as a custom model which is published here (https://huggingface.co/LLM360/CrystalCoder). However, we also want to submit CrystalCoder to the HF leaderboard which requires CrystalCoder to live natively in the transformers library --- thus this PR is created.
Help and guidance would be much appreciated

@TianhuaTao OK, understood! Happy to hear you were able to add the remote code for the model. Let us know when this PR is ready for review 🤗 

Hi @TianhuaTao, 

Re my previous comment, I didn't know about [lighteval](https://github.com/huggingface/lighteval). This library will enable you to evaluate your model on the same benchmarks used for the leaderboard, without having to merge in a model PR. 

What I suggest is running the benchmarks using lighteval. If the model performs significantly well, then we can consider adding it into the transformers library.

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.