[Executorch] Add multithreading for op_quantize

[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

As the title

Differential Revision: D84962233

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

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15609

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❌ 6 New Failures

As of commit e96b3ab with merge base aba44fd ():

NEW FAILURES - The following jobs have failed:

• pull / test-models-linux-basic (vit, portable, cmake, linux.arm64.2xlarge, executorch-ubuntu-22.04-gcc11... / linux-job (gh)
  RuntimeError: Command docker exec -t 61d4270ea6e5f5d784589d041caed1bbb1e89eee456f9cc39a5da48b553cd721 /exec failed with exit code 1
• 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-buck / macos / macos-job (gh)
  The process '/usr/bin/git' failed with exit code 128
• 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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