[Executorch] Add multithreading for op_quantize

[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

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]

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