Fix local repos with remote code not registering for pipelines

[Rocketknight1]

Draft PR, don't merge! Experimenting with fixes for the issue with local repos and remote code not registering correctly for pipelines.

TODO:

• Add a test
• Maybe run some slow autoclass/pipeline tests to make sure I'm not breaking anything
• Try removing model_class.register_for_auto_class(cls.__name__) entirely?

Fixes #32923

[HuggingFaceDocBuilderDev]

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[Rocketknight1]

This is now ready for review! The explanation for it is a bit confusing, though. The original issue is that custom code models did not correctly register with the pipeline model mappings when they were loaded from a local directory instead of from the Hub. The cause is this block of code that treats models differently when they are loaded from a local dir:

if os.path.isdir(config._name_or_path):
    model_class.register_for_auto_class(cls.__name__)
else:
    cls.register(config.__class__, model_class, exist_ok=True)

When I investigated, this code block was added by @sgugger to fix another issue, where custom code models loaded from local directories did not correctly save their modeling files when saved with save_pretrained().

In testing, I tried reverting this change, and it appears that custom code models now correctly save those files even after the revert.

Therefore, this PR reverts the change, and adds two new tests, one test to ensure that loading a local custom code model in a pipeline works now (this test fails without this PR), and one test to ensure that custom code models loaded from a local dir still correctly save their modeling files (this test passes on main, but I'm adding it as an anti-regression precaution).

cc @LysandreJik

[LysandreJik]

Ok the fix works for me and the test is good! As said in the other PR, would it be possible to move the user-owned repos to hf-internal-testing ?

[Rocketknight1]

@LysandreJik done!