Add Mixtral-8x22B-v0.1 model support

[VinitP1102]

Summary

This PR introduces the integration of the Mixtral-8x22B model into the codebase. Specifically, the following changes have been made:

• Added the Mixtral model in mixtral.py to support the 8x22B architecture.
• Introduced a new engine for the Mixtral model in mixtral_engine.py
• Updated __init__.py files in the models and engines directories to register the new Mixtral model.
• Modified generation_config.yaml to include parameters for the Mixtral model's generation tasks.
• Updated finetuning_config.yaml to configure Mixtral model-specific parameters for finetuning.
• Updated documentation, including README.md and supported_models.md, to reflect the addition of the Mixtral-8x22B model, with its identifier key as "mixtral".

Checklist

• Tested the integration of the Mixtral model with both generation and finetuning tasks.
• Updated documentation files to reflect the changes.

Additional Information

• A new example script, mixtral.py, has been added in the examples directory. This script demonstrates how to use the Mixtral model with xTuring and provides instructions for testing the model.
• These changes enable the Mixtral-8x22B model's functionality for both generation and finetuning tasks within the xTuring project. The model's parameters have been incorporated into the respective configuration files.