use ReerCodable macro to allow for default values

[davidkoski]

Proposed changes

Looking at moving to a Codable macro that makes it easier to integrate with JSON configuration files. Specifically:

• easy default values
• easy to override json keys

These are both possible with standard Swift Coding but are unwieldy in the quantity we need here. See:

• https://github.com/ml-explore/mlx-swift-lm/blob/main/Libraries/MLXLMCommon/Documentation.docc/porting.md#configuration

For an example. This changes:

public struct YourModelConfiguration: Codable, Sendable {
    public let hiddenSize: Int

    // use this pattern for values that need defaults
    public let _layerNormEps: Float?
    public var layerNormEps: Float { _layerNormEps ?? 1e-6 }

    enum CodingKeys: String, CodingKey {
        case hiddenSize = "hidden_size"
        case _layerNormEps = "layer_norm_eps"
    }
}

to this:

@Codable
public struct YourModelConfiguration: Sendable {
    @CodingKey("hidden_size") public var hiddenSize: Int
    @CodingKey("layer_norm_eps") public var layerNormEps: Float = 1e-6
}

The downside is that we are using macros (there is some additional build cost).

There was a previous attempt here: ml-explore/mlx-swift-examples#316. This is roughly the same approach but doesn't attempt to port all the configuration at once. Keeping that up-to-date was too much. Once this is in we can migrate the configuration in chunks.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

[davidkoski]

Once #105 is in, this should be a NOP.

[davidkoski]

This is an interesting one because it has some logic to post-process the configuration.

[angeloskath]

Just 🚀

[davidkoski]

This is on hold for now -- I like the approach but am not ready to commit yet.