Add support for ai-forever/mGPT model

[SeuperHakkerJa]

Description

Description: This pull request adds support for the ai-forever/mGPT model. As a multilingual language model based on the GPT-2 architecture, the only additions are its name and alias.

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Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

Checklist:

• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] I have not rewritten tests relating to key interfaces which would affect backward compatibility

[bryce13950]

Thank you for adding this. I will have time to look at this later in the week.