[Bugfix] Fix missing task for speculative decoding

[DarkLight1337]

This PR fixes a bug introduced by #9424 that breaks speculative decoding usage. A special "draft" task is now reserved for models that are designed as draft models.

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can do one of these:

• Add ready label to the PR
• Enable auto-merge.

🚀

[comaniac]

LGTM, although I don't really understand why we introduced "task"

[DarkLight1337]

LGTM, although I don't really understand why we introduced "task"

As mentioned in #9424, some embedding models use the same architecture name as the corresponding text generation model, so we need to specify the task for vLLM to know which worker to initialize.