[Bugfix] Fix failing transformers dynamic module resolving with spawn multiproc method

[Isotr0py]

Issue discussion on Slack: https://vllm-dev.slack.com/archives/C07R5Q1Q2BB/p1739776343893149?thread_ts=1739553140.299949&cid=C07R5Q1Q2BB

• transformers backend failed to load custom module on multiproc executor with VLLM_WORKER_MULTIPROC_METHOD=spawn because false-positive loaded custom module.
• This PR optimize the automap resolving to make sure all custom modules initialized across processes

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[hmellor]

This is an elegant fix, I've just suggested a few modifications to reduce the level of nesting

[hmellor]

LGTM, just add a comment explaining why we need to create all the classes in auto_map (your original one got lost in my suggested refactor)

[tjohnson31415]

This implementation depends on the order of entries in auto_map (eg. how they are iterated). It assumes that the Config class comes before the Modeling class that imports it. That will probably be true in most cases, but we could explicitly load AutoConfig first to be more robust to the ordering.

Another edge case would be if the custom code has relative imports for files that are not in the auto_map, but that seems uncommon...

[hmellor]

This implementation depends on the order of entries in auto_map

Only in the multiprocessing case though, right? Before this PR we never loaded the config class at all (and it is imported by the model class).

[tjohnson31415]

The config class is typically loaded early as part of the engine initialization when the ModelConfig is initialized (using transformers.AutoConfig). It is only in a spawned Worker that we lose the cached import and hit the case that this PR is trying to fix. In that error case, loading the modeling class before its dependencies is what raises the error. If the modeling class is first in auto_map.items(), then it will still fail after this PR.

[Isotr0py]

I think we can sort auto_map from keys to make sure that it has correct like:

{
    "AutoConfig": "<your-repo-name>--<config-name>",
    "AutoModel": "<your-repo-name>--<config-name>",
    "AutoModelFor<Task>": "<your-repo-name>--<config-name>",
}