Make Pyserini optional installation

[16BitNarwhal]

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PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
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  • Description:

Modify setup to make pyserini optional. It's removed from requirements.txt and specified optionally in pyproject.toml so it can be installed with pip install rank-llm[pyserini]