NatashaKnk

Add lowering for iree_attention custom op

2

This PR is a follow-up for [this PR](https://github.com/iree-org/iree-jax/pull/81) in IREE-Jax in an effort to introduce a custom StableHLO attention op which lowers to the linalg_ext version.

This code can (and should) be expanded upon, and would ideally be integrated in the IREE-Jax workflow. At the moment the workflow is broken since Jaxlib has removed ml_program from...