add protocol for array + offset -- to be used by RoPE utils

[davidkoski]

Proposed changes

Adds a protocol:

public protocol OffsetLayer: Module {
    func callAsFunction(_ x: MLXArray, offset: Int) -> MLXArray
}

This will be useful in the RoPE utils in mlx-swift-lm.

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)