Add SnacConfig to enable AutoConfig support for model_type="snac" checkpoints
Author: vaibhav-research
Pull request: yes
Labels: name: Code agent slop

# What does this PR do?

This PR adds a minimal SnacConfig and registers the "snac" model type so that AutoConfig can correctly recognize and load checkpoints with model_type="snac".

This is required for compatibility with exported artifacts (e.g. ONNX checkpoints such as onnx-community/snac_24khz-ONNX) and prevents AutoConfig.from_pretrained(...) from failing with an unrecognized architecture error.

The configuration is intentionally minimal and does not introduce model, tokenizer, or pipeline implementations.

Changes included:
	•	Adds SnacConfig under transformers.models.snac
	•	Registers "snac" in CONFIG_MAPPING_NAMES and MODEL_NAMES_MAPPING
	•	Exposes SnacConfig via the top-level transformers namespace
	•	Adds a fast, offline unit test to ensure AutoConfig.for_model("snac") works as expected



Fixes #43328 


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- [x] Did you write any new necessary tests?


## Who can review?
@Cyrilvallez @ebezzam 

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: auto, snac

View the CircleCI Test Summary for this PR:

https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=43359&sha=78d02c