Metal backend: Add Whisper to CI workflow

[manuelcandales]

This pull request refactors the model export and e2e scripts to support both CUDA and Metal backends, and updates the Metal CI workflow to generalize model export and testing for multiple models and quantization options. It expands CI model coverage to also include Whisper

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15685

• 📄 Preview Python docs built from this PR

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❌ 1 Cancelled Job, 3 Unrelated Failures

As of commit 6d84c80 with merge base 8a8af36 ():

CANCELLED JOB - The following job was cancelled. Please retry:

• trunk / test-models-macos-cpu (llama2, xnnpack-quantization-delegation) / macos-job (gh)
  ##[error]The operation was 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 / unittest / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq
• trunk / unittest-release / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq

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[Gasoonjia]

LGTM!

[mergennachin]

This is great, @manuelcandales

We should also update https://github.com/pytorch/executorch/blob/main/examples/models/whisper/README.md