[Executorch] parallelize op_choose_qparams

[kimishpatel]

Stack from ghstack (oldest at bottom):

• [Executorch][LLM] Use caching allocator for runner #15656
• [Executorch] Make module constructors uniform across #15655
• [Executorch] Use temp allocator for allocating scratch memory #15654
• [Executorch] Introduce caching cpu memory allocator #15611
• Reduce allocation overhead in quantized sdpa #15610
• [Executorch] Add multithreading for op_quantize #15609
• [Executorch] Add simd path for op quantize #15608
• -> [Executorch] parallelize op_choose_qparams #15607
• [Executorch][llm] Fix flakyness of quantized sdpa test #15653

When doing prefill for quantized kv cache, with large prefill length, parallelizing this op helps.

Differential Revision: D84962234

NOTE FOR REVIEWERS: This PR has internal Meta-specific changes or comments, please review them on Phabricator!

[pytorch-bot]

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✅ You can merge normally! (6 Unrelated Failures)

As of commit c69dd4b with merge base 15a0fcd ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• Test Metal Backend / test-voxtral-metal-e2e / macos-job (gh) (matched macos rule in flaky-rules.json)
  File doesn't exist

BROKEN TRUNK - The following jobs failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-binary-size-linux-gcc / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-openvino-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-setup-linux-gcc / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq

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