Add global scale support to quantized layers

[aleroot]

Store optional globalScale on QuantizedLinear and QuantizedEmbedding so nvfp4 weights can preserve the scale needed by lower-level MLX quantize/dequantize operations.

Forward the scale when creating or dequantizing weights, add a direct pre-quantized QuantizedEmbedding initializer to match QuantizedLinear, and guard global-scale execution paths on Metal because MLX does not support globalScale dequantization there.

Add focused tests for the new layer state and parameter exposure without broadening the generic quantization API surface.

Proposed changes

Please include a description of the problem or feature this PR is addressing. If there is a corresponding issue, include the issue #.

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)