[Executorch] Use temp allocator for allocating scratch memory

[kimishpatel]

Stack from ghstack (oldest at bottom):

• [Executorch] make slice_copy parallel #15830
• [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

This allows us to leverage temp memory allocator and if that allocator is caching allocator it reduces the allocaiton overhead.

Differential Revision: D85532076

[pytorch-bot]

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❌ 1 New Failure, 5 Unrelated Failures

As of commit 1d96c89 with merge base 7600df8 ():

NEW FAILURE - The following job has failed:

• pull / test-moshi-linux / linux-job (gh)
  RuntimeError: Could not load libtorchcodec. Likely causes:

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

• Test CUDA Builds / export-model-cuda-artifact (openai, whisper-large-v3-turbo, non-quantized) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128

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

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

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

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