Add Metal backend core ETMetal runtime.

[manuelcandales]

This commit introduces the foundational Metal backend runtime.

Key features:

• ETMetalStream for managing Metal devices, command queues, buffers, and synchronization.
• ETMetalShaderLibrary for compiling Metal shader source and caching pipeline states.
• ETMetalKernelFunction for kernel argument binding, dispatching, and synchronization with stream-managed encoders.
• Added global buffer management and pointer tracking between host and Metal buffers.
• Added global stream management utilities and synchronization helpers

This provides the necessary runtime primitives for executing compute shaders and MPSGraph workloads.

[manuelcandales]

Stack from ghstack (oldest at bottom):

• Add Metal backend build system and runtime integration #15024
• Metal backend: Add operator implementations #15023
• Metal backend: Implement the AOTI MPS shim #15022
• Metal backend: Add AOTI shims for memory management #15021
• -> Add Metal backend core ETMetal runtime. #15020
• Add Metal backend type definitions and utilities #15019
• Add Metal backend Python preprocessing, partitioning, and tests #15015
• [ET][Metal] Update aoti_common with additional AOTI functions needed by Metal backend #15003

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15020

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❌ 3 New Failures, 4 Pending, 1 Unrelated Failure

As of commit 7e971b0 with merge base 896178e ():

NEW FAILURES - The following jobs have failed:

• pull / test-qnn-wheel-packages-linux (3.10) / linux-job (gh)
  RuntimeError: Command docker exec -t 3fb0be8f4f94cf9b345dccf981c7d12e85a22291a4a457cdbc78d7e8e96311ac /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.11) / linux-job (gh)
  RuntimeError: Command docker exec -t fd9e5c5d0770e41492bbe159b00aef0d0ef50360ce44072ff8b5f40c027b5a8e /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.12) / linux-job (gh)
  RuntimeError: Command docker exec -t f4a5d9213e088ea978d4d00b9918e5be0519e79878131aa673c4c3fa5e73f7ec /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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