This kernel wasn't allowed in export, so no backwards compatibility period is required. Even so, the FFI kernels were added 6 months ago.
PiperOrigin-RevId: 724359996
This change does a few things (arguably too many):
1. The key change here is that it fixes the handler registration in `jaxlib/gpu/gpu_kernels.cc` for the two handlers that use the XLA FFI API. A previous attempt at this change caused downstream issues because of duplicate registrations, but we were able to fix that directly in XLA.
2. A second related change is to declare and define the XLA FFI handlers consistently using the `XLA_FFI_DECLARE_HANDLER_SYMBOL` and `XLA_FFI_DEFINE_HANDLER_SYMBOL` macros. We need to use these macros instead of the `XLA_FFI_DEFINE_HANDLER` version which produces a lambda, so that when XLA checks the address of the handler during registration it is consistent. Without this change, the downstream tests would continue to fail.
3. The final change is to consolidate the `cholesky_update_kernel` and `lu_pivot_kernels` implementations into a common `linalg_kernels` target. This makes the implementation of the `_linalg` nanobind module consistent with the other targets within `jaxlib/gpu`, and (I think!) makes the details easier to follow. This last change is less urgent, but it was what I set out to do so that's why I'm suggesting them all together, but I can split this in two if that would be preferred.
PiperOrigin-RevId: 651107659
