Allow graceful handling of cpuinfo init failure

[GregoryComer]

Summary: Don't fatally assert when cpuinfo_initialize fails. This can happen in rare cases on weird hardware. This change instead logs an error and allows execution to continue without the threadpool.

Differential Revision: D74604740

Test Plan:
I locally modified cpuinfo_initialize to always return false. I then attempted to run a model (dl3_xnnpack_fp32.pte) with executor_runner_optimized. With this change, it outputs the following:

E 00:00:00.001021 executorch:XNNPACKBackend.cpp:49] Failed to initialize, XNNPACK status: 0x5
I 00:00:00.087180 executorch:executor_runner_lib.cpp:410] Model file /home/gjcomer/dl3_xnnpack_fp32.pte is loaded.
I 00:00:00.087213 executorch:executor_runner_lib.cpp:420] Running method forward
I 00:00:00.087231 executorch:executor_runner_lib.cpp:470] Setting up planned buffer 0, size 61424384.
E 00:00:00.110105 executorch:method.cpp:78] Backend XnnpackBackend is not available.
F 00:00:00.110144 executorch:executor_runner_lib.cpp:517] In function executor_runner_benchmark(), assert failed (method.ok()): load_me

[pytorch-bot]

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This pull request was exported from Phabricator. Differential Revision: D74604740