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The lowering logic for all jaxlib custom calls are currently split between JAX and jaxlib for reasons that are harder to justify now that the compiled calls are split between jaxlib and the relevant plugins. As part of my project to update these calls and simplify the lowering logic, it makes sense to consolidate the lowering rules in JAX instead of jaxlib since the logic is now the same for both GPU and CPU. This update tackles a simple kernel as a test case for what this would look like. Since the full lowering rule is now implemented in JAX, we can take advantage of the MLIR helpers that are included there, including `jex.ffi.ffi_lowering`, which I needed to update to support shape polymorphism. Of note: I think it is safe (in a compatibility sense) to delete the lowering code from jaxlib, but it does mean that it won't be possible to lower this operation when `jax.__version__ < jaxlib.__version__`. I think this is okay given our compatibility guarantees, but I'd love a sanity check on that! Another note, this doesn't actually change the lowered HLO for this op, so we don't need to worry about export compatibility. PiperOrigin-RevId: 664680250