[BugFix]Fix diffusion profile RPC None results

[yixiaoer]

Fixes #4288

Purpose

The diffusion client currently uses None to mean "RPC result not available yet". Profiling RPCs can also legitimately return None, so successful responses are misread as missing and the request keeps waiting forever. This PR switches the missing-result check to a dedicated sentinel value so /start_profile and /stop_profile can return normally.

Test Plan

• Manual end-to-end validation on a single-node 8 x H100
• Reproduced on tencent/HunyuanImage-3.0-Instruct with a tuned 4+4 multi-stage config:
  • max_num_seqs: 1
  • max_num_batched_tokens: 6144
  • max_tokens: 128
• Compared:
  • an unfixed-minimal control tree where only this sentinel fix was reverted
  • the fixed tree with this patch applied
• Verified both API behavior and profiler output generation on the fixed tree

vLLM Version: 0.22.0

vLLM-Omni Commit: d6ad418

Test Result

• Before this change:

  • the Hunyuan service reached /health = 200
  • POST /start_profile timed out after about 60s with HTTP_CODE=000
  • POST /stop_profile also timed out after about 60s
  • server logs showed the profiler path was entered, but the request never completed

• After this change:

  • the Hunyuan service reached /health = 200
  • POST /start_profile returned 200
  • POST /stop_profile returned 200
  • server logs showed Profiler started., Profiler stopped., and 200 OK for both endpoints
  • profiler traces were exported successfully for both stage-0 AR and stage-1 diffusion ranks

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[RuixiangMa]

@yixiaoer Thans for your contribution, can you also add a ut to cover this?

[hsliuustc0106]

Reviewed PR #4372: [BugFix]Fix diffusion profile RPC None results

[Bounty-hunter]

I tested it and it works, but it returns [[null, null], null], which is quite confusing. Could you fix the response format as well?

[lishunyang12]

Please fix DCO.

[yixiaoer]

Fixed the response format @Bounty-hunter , and also fixed DCO @lishunyang12, thank you.

[lishunyang12]

LGTM