[Deprecate] Deprecate pooling multi task support.

[noooop]

Purpose

Following #37537 #37632

Pooling multitask support is deprecated and will be removed in v0.20.

When the default pooling task is not what you want, you need to manually specify it via PoolerConfig(task=<task>) offline or --pooler-config.task <task> online.

Remove pooling multi task support. #37861

Test Plan

keep ci green

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[mergify]

Documentation preview: https://vllm--37956.org.readthedocs.build/en/37956/

[gemini-code-assist]

Code Review

This pull request effectively deprecates multi-task support for pooling models, encouraging users to specify a single task. The changes are well-structured, with logic centralized in ModelConfig.get_pooling_task and LLM._verify_pooling_task. The documentation and tests have been updated accordingly to reflect this deprecation. I've found one issue in a test case that needs to be addressed.

[noooop]

/gemini review

[gemini-code-assist]

Code Review

This pull request deprecates multi-task support for pooling models, introducing a default pooling task for each model and issuing a warning when a non-default task is used. The changes are well-structured, with updates to configuration, entrypoints, tests, and documentation to reflect this deprecation. The refactoring to centralize task verification logic in _verify_pooling_task is a good improvement. I've found a couple of issues with incorrect error messages that could mislead users, for which I've left specific comments.

[noooop]

cc @DarkLight1337

Ready for review