Add Metal backend CI workflow with Voxtral testing

[manuelcandales]

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[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15233

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❌ 2 New Failures, 7 Unrelated Failures

As of commit e6bbcfc with merge base dbdeb49 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t a945f3ae4c42da3bb85a9fe56566620d977f3f7d0b2d1e06f86333b9a00c08ee /exec failed with exit code 1
• Test Metal Backend / export-voxtral-metal-artifact (gh)
  AttributeError: 'WhisperFeatureExtractor' object has no attribute 'apply_chat_template'

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• docker-builds / docker-build (linux.4xlarge, executorch-ubuntu-22.04-gcc9) (gh) (detected as infra flaky with no log or failing log classifier)
• pull / unittest-editable / linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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

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

• pull / android / run-emulator (gh) (trunk failure)
  The process '/usr/bin/sh' failed with exit code 255
• pull / test-binary-size-linux-gcc / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-openvino-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-setup-linux-gcc / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest / linux / linux-job (gh) (trunk failure)

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