[executorch] Remove EXECUTORCH_CLIENTS visibility gating - Extensions

[GregoryComer]

Stack from ghstack (oldest at bottom):

• [executorch] Remove EXECUTORCH_CLIENTS visibility gating - Devtools, Examples & More #16480
• -> [executorch] Remove EXECUTORCH_CLIENTS visibility gating - Extensions #16479
• [executorch] Remove EXECUTORCH_CLIENTS visibility gating - Other Backends #16478
• [executorch] Remove EXECUTORCH_CLIENTS visibility gating - XNNPACK & Vulkan Backends #16477
• [executorch] Remove EXECUTORCH_CLIENTS visibility gating - Kernels #16476
• [executorch] Remove EXECUTORCH_CLIENTS visibility gating - Runtime (backend/executor/kernel) #16475
• [executorch] Remove EXECUTORCH_CLIENTS visibility gating - Core #16474

Remove the EXECUTORCH_CLIENTS buck client list visibility gating from extension
targets. This replaces all visibility blocks containing @EXECUTORCH_CLIENTS
with explicit PUBLIC visibility.

This is part 6 of a multi-diff stack to remove visibility gating across all of executorch.
This diff covers all extension/*:

• extension/android
• extension/aten_util
• extension/data_loader
• extension/fb (dynamic_shim, ptez)
• extension/flat_tensor
• extension/kernel_util
• extension/llm
• extension/memory_allocator
• extension/module
• extension/pybindings
• extension/tensor
• extension/threadpool
• extension/training

Differential Revision: D89902030

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/16479

• 📄 Preview Python docs built from this PR

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✅ You can merge normally! (2 Unrelated Failures)

As of commit 4d1edb2 with merge base 09b5bdb ():

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / macos / macos-job (gh) (trunk failure)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / android / run-emulator (gh) (#16137)
  Timeout waiting for emulator to boot.

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