add keyword positonal

[IamShubhamGupto]

Proposed changes

Address #1072

(mlx-dev) shubham@Shubhams-MBP mlx % python dummy.py                                   
Traceback (most recent call last):
  File "/Users/shubham/Documents/workspace/forks/mlx/dummy.py", line 5, in <module>
    xent_grad(logits=a, targets=mx.array(0))
ValueError: [grad] Can't compute the gradient of argument index 0 because the function is called with only 0 positional arguments.

Checklist

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