[Misc] Delay EPLB Nixl import until needed

[NickLucche]

Starting a simple colocated vllm instance with a dense model (ep obv disabled) still brings up this unrelated logs that show up when importing nixl

> vllm serve mistralai/Mistral-Medium-3.5-128B -tp 4 --port 8004 --max-model-len 32k
(APIServer pid=377662) INFO 05-06 09:34:02 [utils.py:299]
(APIServer pid=377662) INFO 05-06 09:34:02 [utils.py:299]        █     █     █▄   ▄█
(APIServer pid=377662) INFO 05-06 09:34:02 [utils.py:299]  ▄▄ ▄█ █     █     █ ▀▄▀ █  version 0.20.2rc1.dev69+g66d1cc0c7
(APIServer pid=377662) INFO 05-06 09:34:02 [utils.py:299]   █▄█▀ █     █     █     █  model   mistralai/Mistral-Medium-3.5-128B
(APIServer pid=377662) INFO 05-06 09:34:02 [utils.py:299]    ▀▀  ▀▀▀▀▀ ▀▀▀▀▀ ▀     ▀
(APIServer pid=377662) INFO 05-06 09:34:02 [utils.py:299]
 [ . . . ]
==>(APIServer pid=377662) INFO 05-06 09:34:04 [nixl_utils.py:20] Setting UCX_RCACHE_MAX_UNRELEASED to '1024' to avoid a rare memory leak in UCX when using NIXL.
==>(APIServer pid=377662) INFO 05-06 09:34:04 [nixl_utils.py:32] NIXL is available

(APIServer pid=377662) INFO 05-06 09:34:04 [scheduler.py:239] Chunked prefill is enabled with max_num_batched_tokens=8192.

This PR simply delays importing nixl until needed, that is when eplb with nixl backend has been requested by the user

> vllm serve mistralai/Mistral-Medium-3.5-128B -tp 4 --port 8004 --max-model-len 32k

(APIServer pid=388945) INFO 05-06 09:55:28 [utils.py:299]
(APIServer pid=388945) INFO 05-06 09:55:28 [utils.py:299]        █     █     █▄   ▄█
(APIServer pid=388945) INFO 05-06 09:55:28 [utils.py:299]  ▄▄ ▄█ █     █     █ ▀▄▀ █  version 0.20.2rc1.dev69+g66d1cc0c7
(APIServer pid=388945) INFO 05-06 09:55:28 [utils.py:299]   █▄█▀ █     █     █     █  model   mistralai/Mistral-Medium-3.5-128B
(APIServer pid=388945) INFO 05-06 09:55:28 [utils.py:299]    ▀▀  ▀▀▀▀▀ ▀▀▀▀▀ ▀     ▀
(APIServer pid=388945) INFO 05-06 09:55:28 [utils.py:299]
[ . . .]
(APIServer pid=388945) INFO 05-06 09:55:30 [model.py:563] Resolved architecture: PixtralForConditionalGeneration
(APIServer pid=388945) INFO 05-06 09:55:30 [model.py:1692] Using max model len 32000
(APIServer pid=388945) INFO 05-06 09:55:31 [scheduler.py:239] Chunked prefill is enabled with max_num_batched_tokens=8192.
(APIServer pid=388945) INFO 05-06 09:55:31 [vllm.py:844] Asynchronous scheduling is enabled.
(APIServer pid=388945) INFO 05-06 09:55:31 [kernel.py:212] Final IR op priority after setting platform defaults: IrOpPriorityConfig(rms_norm=['native'], fused_add_rms_norm=['native'])

cc @ilmarkov

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[gemini-code-assist]

Code Review

This pull request refactors vllm/distributed/eplb/eplb_communicator.py by moving the imports of NixlWrapper and nixl_agent_config from the module level into the has_nixl function and the communicator's init method. This change facilitates lazy loading of these dependencies. As there were no review comments provided, I have no additional feedback to offer.

[ilmarkov]

I think there is already PR: #41392 that does similar things

[NickLucche]

@yewentao256 #41392 looks maybe a bit overly complicated, are you sure we don't just need lazy imports for eplb?

[yewentao256]

LGTM, thanks for the work!

I think both of the PRs can be landed as they are doing different things.
This PR fixes the issue for now and previous PR will avoid similiar issues in the future.
@NickLucche Would appreciate your approval for previous PR as well.

[ZhanqiuHu]

LGTM! This is helpful. I think later on if needed we could also wrap the imports in nixl_utils as util functions (e.g. get_nixl_wrapper(), get_nixl_agent_config()) with @lru_cache or something to avoid triggering unnecessary side effects?