Refactor SchedulePolicy to improve code organization

[libratiger]

Motivation

When I try to deep into the Zero-Overhead Batch Scheduler , I find is hard to get clear on the scheduling, and is hard to impl a new scheduling policy, so I try to refactor SchedulePolicy,and make it easy to add new policy for me and others.

Modifications

1. Move sorting logic into separate static methods for better maintainability
2. Improve policy validation and adjustment logic

Testing:

Add new test file test_schedule_policy.py with basic unit tests
Cover policy initialization and FCFS scheduling validation

Checklist

• Format your code according to the Contributor Guide.
• Add unit tests as outlined in the Contributor Guide.
• Update documentation as needed, including docstrings or example tutorials.

[libratiger]

python -m sglang.bench_one_batch --model-path Qwen/Qwen2.5-3B-Instruct

with following result

max_total_num_tokens=1802895
Warmup ...
Prefill. latency: 0.04046 s, throughput:  25310.52 token/s
Decode.  latency: 0.00728 s, throughput:    137.29 token/s
Decode.  latency: 0.00711 s, throughput:    140.58 token/s
Decode.  latency: 0.00711 s, throughput:    140.65 token/s
Decode.  latency: 0.00711 s, throughput:    140.62 token/s
Decode.  latency: 0.00710 s, throughput:    140.76 token/s
Decode.  median latency: 0.00711 s, median throughput:    140.62 token/s
Total. latency:  0.090 s, throughput:  11406.91 token/s
Benchmark ...
Prefill. latency: 0.03336 s, throughput:  30699.61 token/s
Decode.  latency: 0.00716 s, throughput:    139.63 token/s
Decode.  latency: 0.00711 s, throughput:    140.73 token/s
Decode.  latency: 0.00710 s, throughput:    140.84 token/s
Decode.  latency: 0.00710 s, throughput:    140.85 token/s
Decode.  latency: 0.00709 s, throughput:    140.99 token/s
Decode.  median latency: 0.00710 s, median throughput:    140.85 token/s
Total. latency:  0.140 s, throughput:   7431.72 token/s

[libratiger]

cc @merrymercy

[libratiger]

ping @hnyls2002 for your review feedback 😄

[merrymercy]

@hnyls2002 please take a look