Update SDPA to use string "causal" or boolean mask

[awni]

See the relevant part of MLX LM: https://github.com/ml-explore/mlx-lm/blob/ba83487ed9ed78d60fa2a6e3dc5bd0d88d64fc33/mlx_lm/models/base.py#L45-L65

This might need an update to MLX Swift before we can use it, but it will help a lot with prompt processing for long prompts / training.

[davidkoski]

The latest tag of mlx-swift should support the boolean masks. We have another on coming soonish -- maybe I will wait and rev it to the latest if it happens in the next week or so.