Bugfix: Pre-compile EPLB algorithm successfully in subprocess under graph mode

[Mercykid-bash]

This fix addresses the missing pre-compilation of EPLB algorithm in subprocesses under graph mode. With the correction, the kernel pre-compilation procedure runs properly in child processes, eliminating related runtime performance and stability risks.

[gemini-code-assist]

Summary of Changes

Hello @Mercykid-bash, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a targeted warm-up mechanism within the worker process. It ensures that a specific initialization routine is performed for a particular policy type before the worker enters its main processing loop, aiming to optimize performance or resource readiness under those specific conditions.

Highlights

• Conditional Warm-up Execution: A new conditional block has been added to the worker_process method. This block executes a warm-up routine specifically when self.policy_type is set to 3.
• FlashLB Policy Integration: The warm-up function, warm_up(), is dynamically imported from vllm_ascend.eplb.core.policy.policy_flashlb, indicating that this warm-up is tailored for the FlashLB policy.

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[gemini-code-assist]

Code Review

This pull request introduces a warm-up mechanism for the FlashLB policy by adding a warm_up() call in the worker_process. This is a good improvement to ensure Numba JIT-compiled functions are ready before use in the worker process. I have one suggestion regarding code redundancy. Additionally, the pull request title and description could be improved to provide more context, for which I have provided suggestions below as per the repository's style guide.

Suggested PR Title:

[EPLB][Misc] Add warm-up for FlashLB policy in worker process

Suggested PR Summary:

### What this PR does / why we need it?

This PR adds a warm-up call for the FlashLB policy (`policy_type == 3`). The warm-up is performed at the beginning of the EPLB worker process.

This is necessary to pre-compile the Numba JIT-accelerated functions used by the FlashLB policy within the worker process, avoiding performance hits on the first execution.

This change makes the existing `warm_up()` call in `policy_factory.py` redundant. That call runs in the main process and should be removed to avoid unnecessary startup overhead.

### Does this PR introduce _any_ user-facing change?

No, this is a performance optimization for startup and does not affect user-facing functionality.

### How was this patch tested?

CI should pass. Manual verification should show reduced latency on the first balancing operation when using the FlashLB policy.