scheduler.py: Update the name of the default scheduler.

[ryanli]

Purpose

Rectify the name of the default scheduler which has been moved, fixing #26756.

Test Plan

To build an application directly using AsyncLLM using this updated build with default SchedulerConfig.

Test Result

The application starts successfully:

(EngineCore_DP0 pid=64) INFO 10-14 03:58:42 [core.py:210] init engine (profile, create kv cache, warmup model) took 44.50 seconds
INFO 10-14 03:58:55 [loggers.py:147] Engine 000: vllm cache_config_info with initialization after num_gpu_blocks is: 16866

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[gemini-code-assist]

Code Review

This pull request correctly updates the default scheduler class path in vllm/config/scheduler.py, which is a necessary fix to address an application startup failure. While the change is correct for the file modified, it has an unintended side effect in vllm/engine/arg_utils.py that could break configurations for users who explicitly specify the old scheduler path. I have left a detailed comment explaining the issue and a suggested fix. Addressing this backward compatibility issue would make this PR more complete.

[njhill]

Thanks @ryanli