Fix dequantize python sig (dtype default)

[wrmsr]

Proposed changes

The dtype param is missing its None default in its signature, breaking mypy checks against it (as it follows other non-default positionals).

.venvs/default/lib/python3.13/site-packages/mlx/core/__init__.pyi:1674:161: error: Parameter without a default
follows parameter with a default  [syntax]
    ...p_size: Optional[int] = None, bits: Optional[int] = None, mode: str = 'affine', dtype: Optional[Dtype]...
                                                                                        ^
Found 1 error in 1 file (errors prevented further checking)

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)

[awni]

Thanks!