Feat/external architecture registration

[huseyincavusbi]

Hi @jlarson4,

Description

Three changes to allow users to register custom architecture adapters without forking TransformerLens:

1. register_adapter() method — Classmethod on ArchitectureAdapterFactory that adds custom architectures to the registry at runtime
2. Entry-point discovery — discover_entry_points() scans installed packages for adapters declared via transformer_lens.architectures entry group, auto-called on first select_architecture_adapter()
3. Documentation — New page under Contributing with examples for runtime registration, entry-point declaration, and example package layout

Closes #1298

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility

[Copilot]

Pull request overview

Adds a public mechanism for registering custom architecture adapters in TransformerLens without forking the project, plus accompanying tests and documentation.

Changes:

• New ArchitectureAdapterFactory.register_adapter() classmethod for runtime registration, and discover_entry_points() that reads adapters from the transformer_lens.architectures entry-point group on first use.
• New unit test module covering registration, discovery idempotency, and select error paths.
• New documentation page under Contributing describing both registration paths and an example external package layout.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
transformer_lens/factories/architecture_adapter_factory.py	Adds register_adapter and discover_entry_points classmethods; calls discovery at the start of select_architecture_adapter.
tests/unit/model_bridge/test_architecture_adapter_factory.py	New tests for SUPPORTED_ARCHITECTURES contents, runtime registration, error handling, and discovery idempotency.
docs/source/content/contributing.md	Adds the new doc page to the toctree.
docs/source/content/adapter_development/external-adapter-registration.md	New doc page explaining runtime and entry-point registration, with example package layout.

Comments suppressed due to low confidence (1)

tests/unit/model_bridge/test_architecture_adapter_factory.py:58

• This test does not actually verify "overwrite" behavior. It registers MockArchitectureAdapter twice for the same key, so the post-condition cls._adapters[key] is first is trivially true regardless of whether the second register_adapter call overwrote anything. To meaningfully test overwrite behavior, register a second, different adapter class and assert that the registry now points at the new class (and not at first).

    def test_register_overwrites_existing(self):
        key = "TestOverwriteForCausalLM"
        ArchitectureAdapterFactory.register_adapter(key, MockArchitectureAdapter)
        first = ArchitectureAdapterFactory._adapters[key]
        ArchitectureAdapterFactory.register_adapter(key, MockArchitectureAdapter)
        assert ArchitectureAdapterFactory._adapters[key] is first

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