[Model] Explicit interface for vLLM models and support OOT embedding models

[DarkLight1337]

This PR adds interface_base.py which specifies the required methods for vLLM models, something that wasn't clear before. Hope that this can clear up some confusion during model development and refactoring efforts.

I have updated model registry tests to ensure that our existing models satisfy those interfaces.

Based on this change, we now automatically detect embedding models without relying on the internal _EMBEDDING_MODELS global, making them available for OOT - I've added tests for this as well.

[github-actions]

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[youkaichao]

see some nit comments