[Frontend] Remove pooling multi task support. (Hold off until v0.20.0)

[noooop]

Purpose

Following #37537 #37632 #37956

Remove frontend pooling multi task support.

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

Test Plan

Test Result

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[gemini-code-assist]

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[mergify]

Hi @noooop, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

Tip

Is mypy failing?

mypy is run differently in CI. If the failure is related to this check, please use the following command to run it locally:

# For mypy (substitute "3.10" with the failing version if needed)
pre-commit run --hook-stage manual mypy-3.10

[DarkLight1337]

I mean that we should still allow user-specified pooling tasks, but with a warning, until the deprecation period (2 minor versions) is complete, then we can remove it fully

[noooop]

I'm not sure @WoosukKwon is willing to wait two versions, over a month.

[DarkLight1337]

I think we should still honor this policy. That's kinda why he wanted to start the deprecation process ASAP.

[noooop]

I will submit a PR with a warning version. If we're not in a hurry to deprecate it directly, just like this.

[noooop]

Anyway, this PR provides a very solid foundation for the next steps.

[mergify]

Hi @noooop, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

Tip

Is mypy failing?

mypy is run differently in CI. If the failure is related to this check, please use the following command to run it locally:

# For mypy (substitute "3.10" with the failing version if needed)
pre-commit run --hook-stage manual mypy-3.10

[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