executorch [executorch] Migrate extension/memory

Stack from ghstack (oldest at bottom):

#4621
#4620
-> #4619
#4618
#4617
#4616
#4615
#4614
#4613
#4612
#4611
#4610
#4609
#4608
#4607
#4606
#4605

Migrate these headers to the new ::executorch::runtime namespace. Add temporary aliases from the old ::torch::executor namespace so we can migrate users incrementally.

Differential Revision: D60938937

Aug 09 '24 00:08 dbort

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/4619

:page_facing_up: Preview Python docs built from this PR

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:x: 1 New Failure

As of commit 9618d360253287737b15da58d2aa3106e75fb507 with merge base 5c9a00a3e34aa008510989febc42f1de90351494 ():

NEW FAILURE - The following job has failed:

Android / test-llama-app (bpe) / mobile-job (android) (gh) Process completed with exit code 1.

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