fix: handle absent sys.modules entry in modeling_utils

[cjkindel]

What does this PR do?

_can_set_attn_implementation and _can_set_experts_implementation both do a direct subscript lookup into sys.modules:

class_module = sys.modules[cls.__module__]

If the module is not registered under its dotted name in sys.modules — which can happen with lazy loaders, frozen importers, or certain packaging environments on Windows — this raises a KeyError instead of returning False as intended.

The existing comment already acknowledges this class of problem ("This can happen for a custom model in a jupyter notebook or repl for example") but the file guard only ran after the subscript, so it never had a chance to fire.

First introduced: commit 0642963 (PR #42697, merged January 5, 2026).

Real-world traceback reported by Griptape Nodes users:

KeyError: 'transformers.models.clip.modeling_clip'
  modeling_utils.py, in _can_set_experts_implementation
    class_module = sys.modules[cls.__module__]

Reproducing code:

import sys
from transformers.models.clip.modeling_clip import CLIPTextModel, CLIPTextConfig

# Simulate condition where module is absent from sys.modules
del sys.modules["transformers.models.clip.modeling_clip"]

config = CLIPTextConfig()
CLIPTextModel(config)  # KeyError before this fix, succeeds after

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@ArthurZucker (original author)
@Cyrilvallez (model loading)

Closes #45003

[cjkindel]

This isn't slop @Cyrilvallez , this is a legitimate problem affecting Griptape Nodes users with the Advanced Media library. Telling our customers to use my personal fork of Transformers isn't a healthy solution for us, even though it does mitigate the problem.

I acknowledge that Griptape Nodes does some dumb/hacky stuff with sys.modules but this does fix the problem and is 'right'. Without this change, our customers are facing errors when trying to use Flux.1 ControlNets with Diffusers.

[ArthurZucker]

Actually I think vllm had similar issues