Revert "Arm backend: Make composable_quantizer default"

[digantdesai]

Reverts #19758

cc @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell @rascani

[pytorch-bot]

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[Copilot]

Pull request overview

This PR reverts the earlier change that made the Arm backend’s composable quantizer the default, restoring the legacy/default quantizer as the default implementation while keeping the composable quantizer behind an opt-in flag.

Changes:

• Switch use_composable_quantizer defaults back to False across Arm quantizers, update warnings/docs accordingly, and refresh the public API manifest signatures.
• Update Arm tests and tutorial/docs to align with the reverted default (including removing a composable-specific while-loop test).
• Adjust quantization behavior around to.dtype integer→float output qspec handling and update related tests/expectations.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
examples/arm/quantizer_tutorial.ipynb	Marks the composable-quantizer tutorial as WIP/experimental and adds an explicit warning + issue link.
docs/source/backends/arm-vgf/arm-vgf-quantization.md	Updates VGF quantizer default flag and warning text to reflect composable being experimental/opt-in.
docs/source/backends/arm-ethos-u/arm-ethos-u-quantization.md	Updates Ethos-U quantizer default flag and warning text to reflect composable being experimental/opt-in.
backends/arm/test/ops/test_while.py	Removes composable-quantizer-specific while-loop test and related imports.
backends/arm/test/ops/test_transpose_conv2d.py	Updates config type usage to QuantizationConfig and adjusts imports accordingly.
backends/arm/test/ops/test_to_copy.py	Tweaks TOSA INT cast test setup and splits redundant-cast xfails between FP/INT.
backends/arm/test/models/test_torch_functions.py	Simplifies nonzero test wiring and removes the special-case quantizer IO override.
backends/arm/test/misc/test_shared_qspecs.py	Updates expected qspec mappings/counts to match the reverted quantizer behavior.
backends/arm/test/misc/test_quant_custom_meta.py	Removes a quantizer IO override that was tied to the previous default behavior.
backends/arm/quantizer/quantization_config.py	Broadens integer→float to.dtype fixed-output qspec behavior from float32-only to any floating-point output.
backends/arm/quantizer/arm_quantizer.py	Restores default use_composable_quantizer=False, updates warnings/log messaging, and tightens legacy vs composable property behavior.
backends/arm/public_api_manifests/api_manifest_running.toml	Updates public API signatures for EthosU/VGF quantizers to reflect the reverted default parameter value.
backends/arm/_passes/normalize_while_initial_args_pass.py	Changes placeholder meta propagation for cloned placeholders in while normalization.

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