Metal backend: Add Metal int4 quantization support to Parakeet

[manuelcandales]

This PR adds support for 4-bit weight quantization on the Metal backend for Parakeet TDT model.

Parakeet Export Script (export_parakeet_tdt.py, quantize.py)

• Added fpa4w (floating point activation, 4-bit weight) quantization option for encoder and decoder linear layers
• Implemented Metal-specific quantization path using torchao's MPS API (UIntxWeightOnlyConfig)
• Added validation to ensure fpa4w is only used with Metal backend
• Filters out incompatible layers (weights not divisible by 8) during quantization

Documentation (README.md)

• Added fpa4w to quantization config table with Metal backend designation
• Added example showing Metal 4-bit quantization usage
• Reorganized examples to separate CUDA and Metal quantization workflows

CI Integration (export_model_artifact.sh, metal.yml)

• Added quantized-int4-metal option to export script with proper backend validation
• Updated Metal CI workflow to test int4 quantization specifically with parakeet-tdt model

Dependencies

• Bumped torchao pin for latest Metal quantization support

[manuelcandales]

Stack from ghstack (oldest at bottom):

• Metal backend: preserve meta data in decompose_linear_pass #17237
• Metal backend: fail test_modules.py if torchao can't be imported #17236
• -> Metal backend: Add Metal int4 quantization support to Parakeet #17235
• Metal backend: fix bugs in qmv_impl #17188
• Metal backend: import qmv_impl from MLX #17187
• Metal backend: int4 refactor checks and logs #17186
• Metal backend: test 4-bit linear with bias #17127
• Metal backend: enable quantization in test_modules #17118
• Metal backend: support 4-bit linear #17117
• Metal backend: remove parakeet linear decomp #17116
• Metal backend: enable linear with bias #17115
• Metal backend: test modules #17076

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17235

• 📄 Preview Python docs built from this PR

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

❌ 3 New Failures, 2 Pending, 2 Unrelated Failures

As of commit e81b589 with merge base 267a59d ():

NEW FAILURES - The following jobs have failed:

• pull / unittest-editable / linux / linux-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv2_model
• trunk / test-huggingface-transformers-macos (olmo-1b|coreml_fp32_gpu|--quantize) / macos-job (gh)
  The process '/opt/homebrew/bin/git' failed with exit code 128
• trunk / unittest-release / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_linear_model

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

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

• pull / test-samsung-models-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-samsung-quantmodels-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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