Define ET_HAS_EXCEPTIONS macro

[swolchok]

Stack from ghstack (oldest at bottom):

• (to be filled)

To support passing ET_USE_PYTORCH_HEADERS only when exceptions are enabled.

Differential Revision: D76470039

[pytorch-bot]

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❌ 1 New Failure

As of commit 5df49e0 with merge base c4c4763 ():

NEW FAILURE - The following job has failed:

• pull / unittest-arm-backend-with-no-fvp (test_pytest_ops) / linux-job (gh)
  Process completed with exit code 254.

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[swolchok]

double export