Add build-time config for threadpool size

[GregoryComer]

Summary:
Add a preprocessor flag (ET_THREADPOOL_SIZE) and corresponding buck config ("executorch.threadpool_size") to set the default ET threadpool size.

The intent of this change is to allow users in very memory-constrained environments to opt in to a smaller threadpool to shave a small amount of additional memory. Even when measuring resident set size, the addition threads can add more than a Mb of peak memory.

I considered making this a parameter to get_threadpool, but it is not consistently usable, as it depends on who first instantiates the threadpool. As it's a shared resource, it seemed more prudent to make it a build-time configuration.

Differential Revision: D68401795

[pytorch-bot]

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❌ 1 New Failure

As of commit 7ab3449 with merge base fedb035 ():

NEW FAILURE - The following job has failed:

• pull / test-phi-3-mini-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t ed97a216335ba144b8176f6190acef35e5b8021680a52287c71041adfd8c15b7 /exec failed with exit code 2

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This pull request was exported from Phabricator. Differential Revision: D68401795

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