[Misc] Turn MOE_DP_CHUNK_SIZE into an env var

[varun-sundar-rabindranath]

Purpose

There is value in being able to try different MOE_DP_CHUNK_SIZE quickly, without code change,

1. Performance Tuning
2. For large MOE_DP_CHUNK_SIZE and large DP size, we run into OOMs

Long-Term : Based on profile runs, set it to a constant-value / calculate at run-time based on profile runs.

Test Plan

Local Testing on H100.

Test Result

Verified that the value set via the env var is being printed correctly in the logs.

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[gemini-code-assist]

Summary of Changes

Hello @varun-sundar-rabindranath, 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 an environment variable (VLLM_MOE_DP_CHUNK_SIZE) to control the chunk size used in MoE layers when running with Data Parallelism. This change provides flexibility for users to tune performance and manage memory usage (specifically to avoid OOMs) without requiring code modifications, supporting easier experimentation and optimization.

Highlights

• Configurability: The MOE_DP_CHUNK_SIZE parameter, which affects the chunking of tokens processed by MoE layers in Data Parallel mode, is now configurable via the VLLM_MOE_DP_CHUNK_SIZE environment variable.
• Code Cleanup: Removed the hardcoded MOE_DP_CHUNK_SIZE constant from vllm/model_executor/layers/fused_moe/layer.py.
• Usage Update: Updated the MoE layer implementation to read the chunk size from the new environment variable definition in vllm.envs.

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[varun-sundar-rabindranath]

@bnellnm @tlrmchlsmth @abmfy @simon-mo PTAL! Thanks!

[gemini-code-assist]

Code Review

This pull request makes MOE_DP_CHUNK_SIZE configurable via an environment variable, which is beneficial for performance tuning and OOM prevention in MoE models. The changes are well-implemented by updating vllm.envs and replacing the hardcoded constant in the MoE layer. Logging for the new configurable value has also been added.

My primary feedback is to suggest adding input validation for VLLM_MOE_DP_CHUNK_SIZE to ensure it's a positive integer, enhancing robustness against misconfiguration.