Executorch Wasm Internal Build

[Conarnar]

Summary:
Added internal build scripts for Executorch Wasm and unit tests.

Due to some intricacies the following things were done:

• Emscripten build options don't work properly on fbcode, so only xplat targets were made.
• Export scripts are only supported on fbcode, but do not work properly with the Emscripten build mode. Because of this, the test models for unit testing need to be built separately and copied into xplat.
• Jest is configured to not search for unit tests within buck-out, so they must also be copied into xplat.
• These are handled by xplat_build.sh, which can be run with yarn build. These files can also be deleted with yarn clean.
• Unit tests won't build without test_models directory so dummy file is necessary to pass build rule CI.

Differential Revision: D81276482

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14015

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 2 New Failures

As of commit ef31409 with merge base 41c299f ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 0bcddfb9312af58acd2e592f820438aee8a252acda2eef760fc1ea4b53339ce0 /exec failed with exit code 1

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[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D81276482

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[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D81276482