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

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[Copilot]

Pull request overview

This PR implements 4-bit weight quantization support for the Parakeet TDT model on the Metal backend using torchao's MPS API. The changes enable Metal-specific quantization while maintaining existing CUDA quantization workflows.

Changes:

• Added fpa4w (floating point activation, 4-bit weight) quantization option for Metal backend
• Implemented validation to ensure Metal-specific quantization is only used with Metal backend
• Updated CI workflows to test Metal int4 quantization with parakeet-tdt model

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
third-party/ao	Updated torchao submodule to version with Metal quantization support
examples/models/parakeet/quantize.py	Added Metal int4 quantization implementation using UIntxWeightOnlyConfig
examples/models/parakeet/export_parakeet_tdt.py	Added fpa4w option and validation for Metal backend requirement
examples/models/parakeet/README.md	Updated documentation with fpa4w config and Metal quantization example
.github/workflows/metal.yml	Added int4 quantization testing for parakeet-tdt model
.ci/scripts/export_model_artifact.sh	Added quantized-int4-metal option with backend validation

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[mergennachin]

@manuelcandales In the README.md, do you wanna add to run "EXECUTORCH_BUILD_KERNELS_TORCHAO=1 TORCHAO_BUILD_EXPERIMENTAL_MPS=1 ./install_executorch.sh" as a prequisite step for int4 metal quantization?

[manuelcandales]

@manuelcandales In the README.md, do you wanna add to run "EXECUTORCH_BUILD_KERNELS_TORCHAO=1 TORCHAO_BUILD_EXPERIMENTAL_MPS=1 ./install_executorch.sh" as a prequisite step for int4 metal quantization?

yeah, that's true

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

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[mergennachin]

Update the pin past pytorch/ao#3829, and set

uintx_choose_qparams_algorithm="hqq"

[mergennachin]

Could be done in a follow-up PR too

[manuelcandales]

yes, that's my plan, to do in follow-up PR

[manuelcandales]

here #17258