Refactor DeepSeek model by extracting basic functions

[fzyzcjy]

Motivation

In order to support #4068, this PR extracts some small functions from deep seek model and deep ep dispatcher.

The code diff needs to subtract change in #4610, i.e. please view fzyzcjy/sglang@feat/deepseek_async...feat/deepseek_refactor_atomic

Modifications

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
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[fzyzcjy]

Ping me when this PR is to be merged - currently I only resolve conflicts in #4068, and will port the resolve code back here when pinged.

[fzyzcjy]

will reopen by extracting new code (temporarily reset to main branch)