API: Expose base models in API package

[seanggani]

Description

This PR exposes key base classes through the artkit.api module:

• ChatModel
• CompletionModel
• VisionModel
• DiffusionModel

While these classes cannot be instantiated directly, they are frequently used for type hints and this change makes it easier to access them (rather than importing them from their actual .base module)

Issue number and link:

https://github.com/BCG-X-Official/artkit/issues/39

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