[Tracker] XNNPACK BF16 Support

[GregoryComer]

This is the top-level tracking task for support for the BF16 dtype in the XNNPACK delegate. Most of the work is upstream in XNNPACK itself, but we will need to do a small amount of wiring up in the ET delegate once the upstream changes are complete.

Task List:

• Support BF16 for GEMMs, element-wise ops, data movement, and reduction ops in upstream XNNPACK.
  • Tracker: [Tracker] Expanded BF16 Support google/XNNPACK#9728
  • Note that the majority of the work is tracked in the above task.
• Update the ET XNNPACK dependency to pick up the BF16 changes.
• Wire up BF16 support in the ET XNNPACK partitioner and serialization format.
• Wire up BF16 support in the ET XNNPACK runtime.