Single mode skip wait_for_less_busy_worker

[OuYangJinTing]

Description

Great job on the #2079 PR!
#2079 PR added the wait_for_less_busy_worker method, which improves request allocation to avoid being assigned to busy workers. However, for a single model, there are no multiple workers, so there is no need to call the wait_for_less_busy_worker method.

Your checklist for this pull request

• I have reviewed the guidelines for contributing to this repository.
• I have added (or updated) appropriate tests if this PR fixes a bug or adds a feature.
• My pull request is 100 lines added/removed or less so that it can be easily reviewed.
• If this PR doesn't need tests (docs change), I added [ci skip] to the title of the PR.
• If this closes any issues, I have added "Closes #issue" to the PR description or my commit messages.
• I have updated the documentation accordingly.
• All new and existing tests passed, including Rubocop.

PS: English is not my native language; please excuse typing errors.