[Feature Request]: Add Google rerankers as postprocessor

[mdciri]

Feature Description

Add the semantic-ranker-512-003 and semantic-ranker-512-002 (look here) by Google as rerankers postprocessor.

Reason

• I need to test these model in my RAG that works with llama-index.
• In this way Llama-index library is more complete.

Value of Feature

High

[logan-markewich]

@mdciri PRs for this definitely welcome!

[mdciri]

@logan-markewich, I would love to do it, but unfortunately, I cannot spend a lot of time on it and make it perfect enough to pass.

Anyway, I created the PR #18441. I hope you can help me to push it into the main branch.

[mdciri]

@logan-markewich, please, have a look to the open PR and help me to do such integration because unfortunately I cannot put so much time in it