Avoid nested thread-pool locking in parallel_for

[JakeStevens]

Summary:
Using Kleidi can invoke parallel_for inside an existing pool task. Calculating the inner chunk size queries the pool's thread count and relocks the mutex held by the outer call, hanging real model inference even with one thread.

Honor NoThreadPoolGuard before querying the pool. Run nested nonempty ranges directly and preserve the outer thread number used for per-thread scratch storage. Cover nested work, thread-number preservation, and empty ranges in both source mirrors.

AI-assisted: Codex.

Differential Revision: D119433132

[pytorch-bot]

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❌ 5 New Failures

As of commit 795b7d5 with merge base 897fc8e ():

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  >>> Lint for extension/threadpool/test/thread_parallel_test.cpp:

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[meta-codesync]

@JakeStevens has exported this pull request. If you are a Meta employee, you can view the originating Diff in D119433132.

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