Enable async oneDNN build by default

[akhilgoe]

Since XLA's thunk runtime is only compatible with async version of oneDNN, the changes in this PR modify the build pipeline to always build async version whenever oneDNN is compiled.

Note: This only affects the oneDNN build, users still need to switch on the appropriate XLA flags to dispatch to oneDNN kernels:

1. --xla_cpu_experimental_onednn_custom_call : For oneDNN primitive custom calls
2. --xla_cpu_use_onednn : For oneDNN graph (experimental) calls

[gemini-code-assist]

Summary of Changes

Hello @akhilgoe, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request simplifies the build configuration by enabling the asynchronous version of oneDNN by default for mkl_open_source_only setups. This change is essential for maintaining compatibility with XLA's thunk runtime, which relies on this specific oneDNN variant. The update primarily affects the build process, and runtime activation of oneDNN functionality still depends on explicit XLA flags.

Highlights

• oneDNN Async Build Enabled: The build pipeline is now configured to always build the asynchronous version of oneDNN by default when the mkl_open_source_only configuration is active.
• XLA Thunk Runtime Compatibility: This modification is crucial for ensuring compatibility with XLA's thunk runtime, which specifically requires the asynchronous implementation of oneDNN.
• Runtime Flag Requirement: It is important to note that while the build process is updated, users must still explicitly enable oneDNN kernels at runtime using specific XLA flags, such as --xla_cpu_experimental_onednn_custom_call or --xla_cpu_use_onednn.

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[gemini-code-assist]

Code Review

This pull request correctly enables the asynchronous oneDNN build by default by adding the build_with_onednn_async=true flag. This change, applied to the mkl_open_source_only configuration in .bazelrc, is necessary for compatibility with XLA's thunk runtime. The modification is clear, well-justified, and I have no further suggestions.

[agramesh1]

CC @penpornk