cmake: add XLA source fallback for EIGEN_ASYNC

[cfRod]

Description

This change adds an optional source-tree fallback for building the XLA async runtime pieces required by _DNNL_TEST_THREADPOOL_IMPL=EIGEN_ASYNC.

Today the EIGEN_ASYNC test threadpool path requires an XLA CMake package discoverable through find_package(xla CONFIG).
This is difficult to reproduce in open-source development because XLA does not currently provide a documented build or install export for the subset of headers and libxla.a needed by oneDNN’s async Eigen threadpool validation.

The new behavior keeps the existing packaged XLA flow as the preferred path.
If that package is found, oneDNN continues to use the imported xla target.

If no XLA package is found and ONEDNN_XLA_SOURCE_DIR is provided.

This also documents the new build option in the threadpool validation build instructions.

Fixes #5499

Validation

Tested on AArch64 with ACL and Eigen async threadpool enabled.

Configure included:

-DONEDNN_AARCH64_USE_ACL=ON
-DONEDNN_CPU_RUNTIME=THREADPOOL
-D_DNNL_TEST_THREADPOOL_IMPL=EIGEN_ASYNC
-DONEDNN_XLA_SOURCE_DIR=/path/to/xla
-DEigen3_DIR=/tmp/eigen-xla-install/share/eigen3/cmake
-Dabsl_DIR=/tmp/absl-xla-install/lib/cmake/absl

The build produced the fallback libxla.a from:

xla/tsl/platform/logging.cc.o
xla/tsl/concurrency/async_value.cc.o
xla/tsl/concurrency/async_value_ref.cc.o

Also verified the resulting test_reorder binary contains XLA/TSL async symbols such as:

tsl::AsyncValue
tsl::BlockUntilReady
xla::cpu::Worker::Parallelize

Checklist

General

• Do all unit and benchdnn tests (make test and make test_benchdnn_*) pass locally for each commit?
  • Partial: test_reorder passed locally for the validated AArch64 ACL + THREADPOOL + EIGEN_ASYNC build.
• Have you formatted the code using clang-format?

Performance improvements

• Have you submitted performance data that demonstrates performance improvements?
  • Not applicable. This is a build/developer workflow change.

New features

• Have you published an RFC for the new feature?
  • Not applicable. This is an optional build fallback for existing EIGEN_ASYNC validation support.
• Was the RFC approved?
  • Not applicable.
• Have you added relevant tests?
  • Existing EIGEN_ASYNC threadpool tests are enabled by this build path. test_reorder was built and passed with the
    fallback.

Bug fixes

• Have you included information on how to reproduce the issue (either in a github issue or in this PR)?

• Have you added relevant regression tests?

RFC (https://github.com/uxlfoundation/oneDNN/tree/rfcs) PR

• Does RFC document follow the template
  (https://github.com/uxlfoundation/oneDNN/blob/rfcs/rfcs/template.md#onednn-design-document-rfc)?
  • Not applicable.
• Have you added a link to the rendered document?
  • Not applicable.

[dzarukin]

Please update this one, too.

Suggested change

	$ cmake -DONEDNN_CPU_RUNTIME=THREADPOOL -D_ONEDNN_TEST_THREADPOOL_IMPL=EIGEN -DCMAKE_PREFIX_PATH="/path/to/eigen/share/eigen3/cmake;/path/to/absl/lib64/cmake" ..
	$ cmake -DONEDNN_CPU_RUNTIME=THREADPOOL -D_DNNL_TEST_THREADPOOL_IMPL=EIGEN -DCMAKE_PREFIX_PATH="/path/to/eigen/share/eigen3/cmake;/path/to/absl/lib64/cmake" ..

[cfRod]

done

[dzarukin]

Minor: can be probably discarded.

[cfRod]

I kept it since we have touched a new file

[vpirogov]

This one belongs in validation options. Preferably somewhere near _DNNL_TEST_THREADPOOL_IMPL

[dzarukin]

_DNNL_TEST_THREADPOOL_IMPL is right above. :)

[vpirogov]

Oh, looks like I misplaced _DNNL_TEST_THREADPOOL_IMPL...

[cfRod]

Apologies, I was on leave - but is this one still required? I have addressed the other comment