Add BLEU metric for MT task

[sky-2002]

Pull Request Template

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Description

This PR adds the BLEU metric for MT tasks.

Fixes #278

Checklist

• I have read the CONTRIBUTING document.
• My code follows the code style (BLACK) of this project.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the documentation accordingly.
• I have added an appropriate CHANGELOG entry.

Author's Note

I have tested my code by only running the test_bleu_operator from the tests/test_oprators.py file.