[Executorch] parallelize op_choose_qparams

[kimishpatel]

Stack from ghstack (oldest at bottom):

• [Executorch][LLM] Use caching allocator for runner #15730
• [Executorch] Make module constructors uniform across #15729
• [Executorch] Use temp allocator for allocating scratch memory #15728
• [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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❌ 4 New Failures

As of commit 781f1ae with merge base aba44fd ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / linux / linux-job (gh)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_int8_static_quant_recipe
• pull / unittest / macos / macos-job (gh)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_int8_static_quant_recipe
• pull / unittest-editable / linux / linux-job (gh)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_int8_static_quant_recipe
• pull / unittest-editable / macos / macos-job (gh)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_int8_static_quant_recipe

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