[Model][7/N] Improve all pooling task | Deprecation as_reward_model. Extract hidden states prefer using new multi-vector retrieval API

[noooop]

Purpose

Can you imagine using llm.reward to extract hidden states? I just found out today.

Fix #11905

Test Plan

Test Result

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[gemini-code-assist]

Code Review

This pull request deprecates and removes the as_reward_model adapter, along with its usage in tests and model loading utilities. The changes are straightforward and correctly implemented. This refactoring improves the API by removing a generic adapter that was used for extracting hidden states, encouraging more explicit methods. The code modifications are sound and I have no specific comments on the changes.

[chatgpt-codex-connector]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @noooop.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork